1. Introduction:

The gospel of Thomas is a collection of alleged Jesus' sayings (logions). We have two versions of the (uncanonical) gospel today. The first was discovered in the late 1800's among the Oxyrhynchus Papyri, and consists of fragments of a Greek version (GrGTh), one of those (Oxy 1) dated 200C.E. at the earliest, for paleographical reasons. The second is a complete version (CoGTh) in Coptic, found among many "Gnostic" texts from Codex II of the Nag Hammadi finds and dated about 340C.E. Scholars generally agree that Thomas' gospel was first written in Greek, likely in Syria.
The fragmentary Greek version includes a short prologue and logions 1 to 6, 27, 28, 30 to 32, 36, 37 and one not appearing in CoGTh. Furthermore logion 30 incorporates also an element from CoGTh logion 77.
The (complete) Coptic version has the prologue and logions 1 to 114. The last logion is undoubtedly Gnostic and considered by many scholars to be a late addition (I will be gracious on that one).
Important remark: my dating of GThomas (100-130) should not be construed as indicative of the time when a Thomassan sect started to appear. As a matter of fact, through my studies, not only of GThomas, but also of the making of GJohn, I am inclined to think this particular sect existed as early as the 70's. However the gospel of Thomas is representative of the sectarians' beliefs in 100-130 only, while some of those beliefs (such as the realized eschatology) were not existing earlier on (see explanation later on this page).

Note: I use the translations by Thomas O. Lambdin (Coptic version)
B.P Grenfell & A.S. Hunt (Greek Fragments)
Bentley Layton (Greek Fragments)

The most reliable external evidence about the gospel of Thomas is found into the works of Hippolytus of Rome ('The Refutation of all Heresies', V, II). Writing between the years 222-235C.E., Hippolytus quoted a saying, a loose combination of logions 4, 5, 94 & an aphorism from Hippocrates:
"And concerning this (nature) they [the Naasseni, a Gnostic sect, which started around 120-140C.E.] hand down an explicit passage, occurring in the Gospel inscribed according to Thomas, expressing themselves thus:
[suggesting the quote is a Naassene's extrapolation]
` "He who seeks me, will find, me in children from seven years old; for there concealed, I shall in the fourteenth age be made manifest."
This, however, is not (the teaching) of Christ, but of Hippocrates, who uses these words: "A child of seven years is half of a father." And so it is that these (heretics), placing the originative nature of the universe in causative seed, (and) having ascertained the (aphorism) of Hippocrates, that a child of seven years old is half of a father, say that in fourteen years, according to Thomas, he is manifested."

But even earlier, '2Clement' (written 140-160) provides a quote as "the Lord himself ... said" (12:20) which seems to be extracted from the gospel of Thomas (GThomas), as shown later in this page.

Many GThomas logions have parallels in the canonical gospels, as shown below:
Please note the following classifications cannot be definitive, being subject of judgment calls & opinions.
a) Close Parallels
9: Mk4:3-8, Mt13:3-8, Lk8:5-8 10: Lk12:49 16: Mt10:34-36, Lk12:51-53 20: Mk4:30-32, Mt13:31-32, Lk13:18-19 26: Mt7:3-5, Lk6:41-42 34: Mt15:14, Lk6:39 35: Mk3:27, Mt12:29, Lk11:21-22 41: Mt25:29, Lk19:26 45: Mt7:16-20, Lk6:43-46 46: Mt11:11, Lk7:28 54: Mt5:3, Lk6:20 57: Mt13:24-30 63: Lk12:16-21 64: Mt22:3-9, Lk14:16-24 65: Mt21:33-39, Mk12:1-8, Lk20:9-15 66: Mk12:10, Mt21:42, Lk20:17 73: Mt9:37-38, Lk10:2 86: Mt8:20, Lk9:58 89: Lk11:39-40 93: Mt7:6 94: Mt7:7, Lk11:9 96: Mt13:33, Lk13:20-21, 100: Mk12:13-17, Lk20:22-25 103: Mt24:43, Lk12:39 104: Mk2:19-20, Mt9:14-15, Lk5:34-35 107: Mt18:12-13, Lk15:3-7
(total: 26)
b) Remote Parallels
1: Jn8:51 2: Mt7:7, Lk11:9 8: Mt13:47-48 17: 1Co2:9, Isa64:4 30: Mt18:20 31: Mk6:4, Lk4:23-24, Jn4:44 32: Mt5:14b 36: Mt6:25, Lk12:22 40: Mt15:13 44: Mk3:28-29, Mt12:32, Lk12:10 59: Lk17:22, Jn7:34,13:33 68: Mt5:11, Lk6:22 71: Mk14:58 72: Lk12:13-14 75: Mt22:14 78: Mt11:7-9, Lk7:24-26 90: Mt11:28-30 95: Lk6:34-35, Lk14:12-14 99: Mk3:31-35, Mt12:46-50 101: Mt10:37, Lk14:26 109: Mt13:44
(total: 21)
c) Multiple Parallels
14a: no parallels 14b: Lk10:8-9 14c: Mk7:15, Mt15:11 21a: no parallel 21b: Mt24:43, Lk12:39 21c: Mk4:29 22a: Mt18:3, Lk18:17 22b: Mk9:43-48, Mt5:29-30 24a: Jn13:36 24b: Mt6:22-23, Lk11:34-36 33a: Mt10:27 33b: Mk4:21, Mt5:15, Lk8:16,11:33 39a: Lk11:52 39b: Mt10:16 43a: Jn:8:25 43b: Mt7:16-20, Lk6:43-46(?) 47a: no parallels 47b: Mt6:24, Lk16:13 47c: Lk5:39 47d: Mk2:22, Mt9:17, Lk5:37-38 47e: Mk2:21, Mt9:16, Lk5:36 55a: Lk14:26 55b: Mt10:37 62a: Mk4:11, Mt13:11, Lk8:10 62b: Mt6:3 69a: Mt5:8 (cf. Thomas saying 68) 69b: Mt5:6, Lk6:21 76a: Mt13:45-46 76b: Mt6:20, Lk12:33 79a: Lk11:27-28 79b: Lk23:29 91a: Jn9:36 91b: Lk12:54-56
(total: 14)
d) Partial Parallels
3: Lk17:21 4: Mk10:31, Mt19:30,20:16 5: Mt10:26, Lk12:27 6: Mt10:26, Lk12:2 38: Lk17:22, Jn7:34,13:33 48: Mk11:23-24, Mt18:19 58: Mt11:28 92: Mt7:7, Lk11:9
(total: 8)

More than half of the Thomassan logions have undeniable similarities with verses from the canonical gospels.

Many of these logions are particularly obscure and do not make any human sense. Here are two examples (see partial parallel in bold):
GrGTh 4 "Jesus said: "Let the old man who is full of days not hesitate to ask the child of seven days about the place of life; then he will live. For many that are first will be last, and last, first, and they will become a single one.""
CoGTh 7 "Jesus said, "Blessed is the lion which becomes man when consumed by man; and cursed is the man whom the lion consumes, and the lion becomes man.""

Note: however logion 7 can be explained very well as an elaboration of a verse (which makes sense) from 1Peter (written around 80):
5:8 "Be sober, be vigilant; because your adversary the devil walks about like a roaring lion, seeking whom he may devour."
and the absence of "devil" in the logion is justified by the fact GThomas never mentions Satan, devil or any entity of that nature.

Many arguments have been put forward in favor or against GThomas early (pre-gospels) or late (post-gospels) writing. Rather than get involved into some seemingly endless general discussions/debates fully addressed in other web sites, I will first look at the prologue and five specific logions in order to make my case for late writing. Doing so, I ask my readers to heed to my accompanying comments about the composition of the GTh logions. It is because their simple (so-called "primitive") wording is one main argument for early writing. I will come back to that later, in the next chapter.

2. Prologue and five logions

2.1 Prologue:
GrGTh: P "These are the secret sayings which were spoken by Jesus the Living One, and which Judas, who is called Thomas, wrote down"
CoGTh: P "These are the secret sayings which the living Jesus spoke and which Didymos Judas Thomas wrote down."

Notes:
a) The Coptic word for 'secret' can (and is) also translated by "hidden".
b) Right from the start, let's notice the difference in wording between the Greek version and the later Coptic one.
c) Didymos (or didymus) is 'twin' in Greek. 'Thomas' means 'twin' in Aramaic.

A) Thomas:
Thomas is also the name of one of the twelve apostles, according to all the gospels and 'Acts':
Mk3:18
Mt10:3
Lk6:15
Ac1:13
Jn11:16,14:5,20:24,26,27,28,29,21:2
We notice how important Thomas is in the last gospel, as compared with only one mention in each of the previous ones.
In John's gospel, only Peter (32) and Philip (12) are named more often than Thomas (8); Andrew gets 4 mentions, Matthew is ignored, John & his brother James, the fishermen, are not mentioned separately by name.
In Mark's gospel, Peter is named 19 times, Andrew 4 times, John 12 times and John's sibling, James, 11 times.
The later importance of Thomas is also reflected in Eusebius, 'The History of the Church', 3, 39, "the writings of Papias":
(Papias was a late 1st century to early 2nd prominent Christian of Asia Minor)
"And whenever anyone came who had been
[let's notice the past tense: the follower is alive then but the presbyters are likely deceased]
` a follower of the presbyters, I [Papias] inquired into the words of the presbyters [elders], what Andrew or Peter had said, or Philip or Thomas or James or John or Matthew, or any other disciples of the Lord ..."
Thomas is named right after Andrew (as Peter's brother), Peter & Philip, and before James, John & Matthew, corroborating the "ranking" of Thomas in GJohn.

Notes:
a) Papias is getting third hand what the aforementioned disciples allegedly said. As an example:
Jesus > Thomas > elder > follower > Papias
In Parables and gospels, Section 7, I quoted a surviving passage from Papias' works, showing the "followers" (or Papias!) were very much inspired by the gospels and 'Revelation' in transmitting the oral "tradition".
b) The "talent" and drive of latter alleged "follower(s)" might have been a lot more determinant than the real stature of the long ago deceased disciple! That would explain why Thomas, not included by Paul as a "pillar" of the church of Jerusalem (unlike Peter & John, Gal2-9), became prominent later on (with Philip), as in GThomas itself:
CoGTh 13 "Jesus said to His disciples, "Compare me to someone and tell Me whom I am like."
Simon Peter said to Him, "You are like a righteous angel."
Matthew said to Him, "You are like a wise philosopher."
Thomas said to Him, "Master, my mouth is wholly incapable of saying whom You are like."
Jesus said, "I am not your master. Because you have drunk, you have become intoxicated by the bubbling spring which I have measured out." And He took him [Thomas] and withdrew and told him three things. When Thomas returned to his companions, they asked him, "What did Jesus say to you?" Thomas said to them, "If I tell you one of the things which he told me, you will pick up stones and throw them at me; a fire will come out of the stones and burn you up.""

B) Judas Thomas:
In the synoptic gospels and 'Acts', Thomas is just "Thomas". In GJohn, he is "Thomas, called the twin (didymos)" three times (Jn11:16;20:24;21:2).
In the whole known 1st century Christian literature, let's note there is no mention of 'Judas=Thomas'. But in the 2nd to 4th century Christian writings, this Thomas becomes called 'Judas Thomas' ('Thomas the Contender' 150-225, 'Acts of Thomas' 200-225, 'Teaching of Addai' and 'the Agbar legend' 325). In the first book, Judas Thomas is declared to be the twin brother of Jesus!

Note:
Jn14:22 "[at the "Last Supper"] Judas (not Iscariot) said to Him, "Lord, how is it that You will manifest Yourself to us, and not to the world?""
Some scholars suggested this 'Judas', appearing only in this verse, was no less than Thomas himself. But that is most unlikely, because:
a) Nowhere in the gospel, "John" associated the two names, such as 'Judas, called Thomas'. Why would the author suddenly refer of Thomas as "Judas", prompting the clarification this 'Judas' is not Iscariot (and NOT: "called Thomas")?
b) This "Judas" could have been one of the deceased disciples, "represented" one or two generation(s) later by an active alleged follower (as per Papias' testimony). Let's also note that in
Lk6:13-16 "... He also named [12] apostles: Simon, whom He also named Peter, and Andrew his brother; James and John; Philip and Bartholomew; Matthew and Thomas; James the son of Alphaeus, and Simon called the Zealot; Judas the son of James, and Judas Iscariot who also became a traitor."
a second 'Judas', a certain "Judas the son of James" has replaced Thaddaeus from the list of the twelve provided earlier by "Mark" (3:1-19).

In conclusion, the "Judas, who is called Thomas" suggests that GThomas was composed either during (according to the importance given to Thomas) or after (according to Thomas being also called Judas) the writing of GJohn (from 80 to 105; see here for justifications).

PS: for a dating towards the 2nd century, even more evident is the following, about the only named disciples in GThomas, that is James, Peter, Matthew, Thomas/Judas, Mary and Salome (logions 13, 21 & 61):
A) About Judas, Matthew, Mary & Salome:
a) A certain 'Judas' (never called Iscariot or hostile to Jesus) is also featured in the 'Dialogue of the Savior', a Gnostic writing usually dated 120-180, which also has parallels with the canonical gospels (& GThomas). The only followers named here are Matthew (named 17 times), Judas (20) and Mary (14) (but no Peter!).
b) Also dated 120-180, the Gnostic 'Gospel of Mary' features Peter (7), Andrew (1), Levi/Matthew (1) and, of course, Mary (6), as the only named followers of Jesus and "[Levi to Peter] the Savior knows her very well. That is why He loved her more than us."
c) Around 120, Papias reported about a 'Matthew', as the compiler of 'Logias'. Before that, Matthew is just one minor disciple among the twelve (according to the Synoptics), not one of the four close associates of Jesus (two sets of brothers, Peter & Andrew and John & James), not one of the three named "pillars" (Gal2:9) and unmentioned in GJohn.
d) In all of the 1st century Christian texts, 'Salome' is mentioned only in GMark, as a distant follower of Jesus, among other women. Even GLuke & GMatthew do not bother to reproduce her name. But in 2nd century Christian writings, Salome has become a close disciple of Jesus, as evidenced in the 'Gospel of the Egyptians' (100-150?) (& also mentioned in the 'Infancy gospel of James' (140-170) where she confirms Mary's virginity (after the birth) by "testing"). This is mirrored in Logion 61 of GThomas where she becomes a close disciple.
e) Concluding the four points above, 'Matthew', as also 'Judas', 'Mary' with NO 'Magdalene' attached and 'Salome', became prominent in the 2nd century, but not before that. And because 'Matthew' and 'Salome' appear in GThomas, as two out of the six named disciples (including James & Mary), that would point to this book being written/compiled in the 2nd century.
B) More about Mary:
a) In the gospels (written 70-105), all the 'Mary', including Mary Magdalene, are clearly identified with an attribute. However, in Jn20:11&16, only 'Mary' is written, but the context, and more so Jn20:18 (which specifies this 'Mary' being Mary Magdalene), removes any confusion. But the 'Mary' (without 'Magdalene') in Jn20:11 & Jn20:16 (here named by Jesus himself!) may very well be the precedent which started Mary Magdalene as being called just 'Mary' in 2nd cent. writings!
b) Also, Mary Magdalene is depicted in the Synoptics as only a distant follower, among other women (Mk15:40-41, Mt27:55-57, Lk8:1-3). But in GJohn, Mary Magdalene is singled out, and honored as the first one to see the resurrected Jesus, and promoted as 'the daughter of God' (Jn20:14-17). More, she calls Jesus "rabboni" (Jn20:16), an expression for "my teacher", implying she is a close disciple. That would certainly open the way for the treatment of Mary in Logion 21 (as a close confidante of Jesus) and in the 'Gospel of Mary' (where she is the disciple that Jesus loves best)!
C) About James:
a) In logion 12, James is said to be the pre-selected replacement for Jesus, and to whom the disciples (including Peter & Thomas) should go to as their new leader.
But according to 'Acts', James (not a follower of Jesus according to the gospels) became a leader of the church of Jerusalem rather progressively, after some years.
However, in the early 2nd century, the 'Apocryphon (or secret book) of James', dated 100-150, has James not only as a "Gnostish" Christian & Jesus' preferred disciple (with Peter), but also as the boss of the twelve, all of that before Jesus' final ascent to heaven. Also to be considered: the 'Gospel of the Hebrews', written around 100, also has James as a follower; furthermore, he is the beneficiary of a post-mortem reappearance by his brother. In conclusion, these two later aforementioned writings would certainly clear the way for James to be for the disciples the pre-assigned Jesus' successor in GThomas!
b) In GThomas, James is called James the just (or righteous). No 1st century texts we know of call him that way. However in the 2nd century, the aforementioned 2nd Apocalypse of James and Hegesippus' description of James' death (written c.a. mid-century) qualify James as 'the just'.
c) Let's also say that in the Apocryphon, James is writing a "secret" book "to you - and to you alone ... take care not to recount this book to many" (explaining why the book was not known before being "discovered" generations later!). And all the disciples (that would includes Thomas) are also writing their own book, "remembering what the Savior had said to each one of them, whether secretly or openly"! That would open the way for "secret sayings" written by Thomas!
And why would Thomas himself write the sayings without the intention to publish them right away (and making sure of it)?
But if known then (during 30-60), they could not be qualified as secret (hidden)!
However these same sayings could be easily said 'hidden' if published way later, with the understanding they were (allegedly!) transmitted orally by individuals along several generations, as from the writings of Papias (110-140), previously quoted:
"And whenever anyone came who had been a follower of the presbyters, I [Papias] inquired into the words of the presbyters [elders], what Andrew or Peter had said, or Philip or Thomas or James or John or Matthew, or any other disciples of the Lord ..."
and as commented further by Eusebius ('The History of the Church', 3, 39):
"Papias reproduces other stories communicated to him by word of mouth, together with some otherwise unknown parables and teachings of the Savior"

Remarks:
a) The above postscript is not meant to "prove" GThomas is a 2nd century composition, but certainly, it strongly points to that time period, and from many paths ('Judas', 'Matthew', 'Mary', 'Salome', 'James', 'the just' , "secret" and Papias' writings).
b) The 2nd century 'Gospel of Mary' (GMary) and the 'Apocryphon of James', not only use the authority of Mary & James, but also, within the text, elevate their status above the other disciples. These Christian writings (and many others from the same century) refute the following main argument for a 1st cent. dating:
As Patterson noted, "the collection must come from a period in which particular communities were still appealing to the authoritative position of particular apostles as a way of guaranteeing the reliability of its traditions. The insipid and the title certainly function in this way. But one might also point to Thom 12, which appeals to James, and to Thom 13, which appeals to the authority of Thomas, to illustrate the feature. In this sense the Gospel of Thomas is comparable to Matthew,
[but also GMary, the 'Apocryphon of James' and the 'Dialogue of the Savior', all 2nd century writings!]
` in which the authority of Peter is asserted (Matt 16:13-20), or perhaps to the deutero-Pauline epistles, which appeal to Paul's authority in like manner. All of these texts derive from the last decades of the first century C.E.
[what about the "Pastorals" (1-2 Timothy & Titus) and 2Peter, all written in the first half of the 2nd cent.?]
` It should be noted that Thomas does not appeal to the authority of Thomas or James simply because they are "apostles." Thomas never treats "the twelve" as a rarefied concept, a venerated group.
["disciples" appears 19 times in CoGThomas (but never (the) "twelve").
Other 2nd century books:
'Apocryphon of James': "disciples"=3, "twelve"=2, named disciple=10
GMary: "disciples"=0, "twelve"=0, named disciple=15
'Dialogue of the Savior': "disciples"=9, "twelve"=1, named disciple=51
A typical 1st century book:
GMark: "disciples"=45, "twelve"=11, named disciple=45
I do not see the pattern Patterson is talking about!]
` The authority of each apostle is not taken for granted by virtue of the status earned by simply through being part of "the twelve."
[but in the 2nd cent. 'Apocryphon of James', James (not one of the twelve) & Peter are separated from the other disciples, to become the direct recipients of important secrets from the resurrected Jesus]
` In Thomas 13, for example, Thomas' answer is up as exemplary, but those of Peter and Matthew are deprecated as inadequate
[but in GMary, the apostles themselves, more so Peter, are deprecated:
"He [Peter] questioned them [the twelve] about the Savior: Did He really speak privately with a woman and not openly to us? Are we to turn about and all listen to her? Did He prefer her to us?"
"He loved her more than us."
"Levi answered and said to Peter, Peter you have always been hot tempered"] ..."
(GThom & Jesus, p. 116)

2.2 Logion 1:
Let's examine the following, starting by GJohn, the suspected origin of the saying.
Jn8:52b "'If anyone keeps My word he shall never taste death.'"
CoGTh 1 "And He said, "Whoever finds the interpretation of these sayings will not experience death.""
In the whole N.T., the notion of "'If anyone keeps My word he shall never taste death.'" is found only in (the original) GJohn, and more than once:
Jn8:51 "Most assuredly, I say to you, if anyone keeps My word he shall never see death."
Jn10:27-28a "My sheep hear My voice, and I know them, and they follow Me. And I give them eternal life, and they shall never perish; ..."
with some variation:
Jn3:15 "... whoever believes in Him [the Son] should not perish but have eternal life."
And because the other gospels do not describe Jesus as likely to utter anything resembling those, the authenticity of these sayings is very questionable:
Lk12:25-26 "And which of you by worrying can add a single hour to his life's span? If then you cannot do even a very little thing, why do you worry about other matters?"

Furthermore, the concept of "anyone ... [the faithful] shall never see death" was preached by "heretics". According to Eusebius, 'The History of the Church' 3, 26, (placing Menander in the 70-117C.E. period), quoting Justin Martyr, a mid-2nd century Christian, 1Apology, 26:
"Another Samaritan, called Menander, from the village of Caparattaea, became a disciple of Simon and like him was driven mad by demons. It is known that he arrived in Antioch and deluded many by magical trickery. He even persuaded his followers that they would not die: and there are still some [generations later!] who on the strength of his assertion maintain this belief."
And from Irenaeus, 'Against Heresies', I, 23:
"his disciples [Menander's ones] obtain the resurrection by being baptized into him, and can die no more, but remain in the possession of immortal youth"
Was "John" influenced?
Likely so, but he changed his mind later (see here about the progressive making of the gospel).

But more so the writer(s) of GThomas:
Here, 'death' means real death as in the secular meaning:
CoGTh 85 "Jesus said, "Adam came into being from a great power and a great wealth, but he did not become worthy of you. For had he been worthy, [he would] not [have experienced] death.""
As an answer on how some lives will end, GThomas is very evasive again:
CoGTh 18 "The disciples said to Jesus, "Tell us how our end will be." Jesus said, "Have you discovered, then, the beginning, that you look for the end? For where the beginning is, there will the end be. Blessed is he who will take his place in the beginning; he will know the end and will not experience death.""
On the subject of death, there is more:
CoGTh 59 "Look to the Living One as long as you live, lest ['in order not' and NOT "because when"] you die, then search [as a spirit?] for him, and fail."
CoGTh 111 "Jesus said, "The heavens and the earth will be rolled up in your presence [Mk13:31?]. And one who lives from the Living One will not see death." ..."

In conclusion, the GThomas writer(s) offered hope for immortality on earth (belief started by Menander as soon as the 70's and still existing among some Christians in the mid 2nd cent.) by keeping the faith and rightly understanding mysterious logions, or even finding the earthly Paradise:
CoGTh 19 "Jesus said, "Blessed is he who came into being before he came into being. If you become My disciples and listen to My words, these stones will minister to you. For there are five trees for you in Paradise which remain undisturbed summer and winter and whose leaves do not fall. Whoever becomes acquainted with them will not experience death.""
The Kingdom (with its associated eternal life & which had not come overtly yet), a problem for other Christian leaders, was solved: it is here, on earth! But 'finding & entering' was through correctly decoding cryptic sayings (as the ones quoted already in the section 'Logion 3'), including unexplained parables.
And the second coming had happened already, sort of:
GrGTh 30/77 "Jesus said: "Where there are [two, they are not] without God, and when there is one alone, [I say,] I am with him. Raise the stone, and there you will find me; cleave the wood, and there I am.""

PS: more about the possibility of "Thomas" knowing about GJohn. Let's consider:
Jn14:5-7a "Thomas said to Him, "Lord, we do not know where You are going, and how can we know the way? Jesus said to him, "I am the way, the truth, and the life. No one comes to the Father [in heaven!] except through Me. If you had known Me, you would have known My Father also ..."
Here the answer of the question is very clear: in order to go to heaven (that is to the Father), one has to acknowledge (& follow) the Son. In GThomas, a similar question is asked, but the answer is extremely indefinite & evasive. Then, why would "Thomas" bother to spell out a question, to which he obviously has no answer, if not in order to counteract Jn14:5-7a (suggesting the answer there is not true)?
CoGTh 24 "His disciples said to Him, "Show us the place where You are, since it is necessary for us to seek it." He said to them, "Whoever has ears, let him hear. There is light within a man of light, and he (or "it") lights up the whole world. If he (or "it") does not shine, he (or "it") is darkness.""
And what about "John" knowing about the Thomassan sectarians and their beliefs then?
Definitively Yes! And in this part of GJohn (written around 90-95, when the author was dropping the notion of "NOT dying"), "John" had Thomas himself not afraid, even willing, to die (we are far away from the expectation of avoiding death!):
Jn 11:16 "Then Thomas, who is called the Twin, said to his fellow disciples, "Let us also go, that we may die with him."
And in the 'doubtful Thomas' passage (Jn20:24-29), Thomas himself witnesses the resurrected Jesus (the example of future resurrections and proof of the divinity of Jesus, both not believed by the "Thomassans"!) and then declares "My Lord and my God" (not just a wise teacher/prophet!).

2.3 Logion 57:
CoGTh 57 "Jesus said, "The Kingdom of the Father is like a man who had [good] seed. His enemy came by night and sowed weeds among the good seed. The man did not allow them to pull up the weeds; he said to them, 'I am afraid that you will go intending to pull up the weeds and pull up the wheat along with them.' For on the day of the harvest the weeds will be plainly visible, and they will be pulled up and burned.""
The same parable appears also in the much longer Matthew's version:
Mt13:24-30: "Another parable He put forth to them, saying: "The kingdom of heaven is like a man who sowed good seed in his field; but while men slept, his enemy came and sowed weeds
[I replaced all 'tares' from the NKJV translation by 'weeds']
` among the wheat and went his way. But when the grain had sprouted and produced a crop, then the weeds also appeared. So the servants of the owner came and said to him, 'Sir, did you not sow good seed in your field? How then does it have weeds ?' He said to them, 'An enemy has done this.' The servants said to him, 'Do you want us then to go and gather them up?' But he said, 'No, lest while you gather up the weeds you also uproot the wheat with them. 'Let both grow together until the harvest, and at the time of harvest I will say to the reapers, "First gather together the weeds"
[impossible to do without thrashing the wheat]
` and bind them in bundles to burn them, but gather the wheat into my barn.'
[no problem! As long as you spend a lot more money to pay the harvesters and you are happy with a drastically reduced crop. The author of this parable could not have been familiar with farm work, or cared more about theological implications than facts]"
We notice the GThomas version is more streamlined, with less superfluous details and better written.
In "Parables and gospels", I took great pain to explain that the parable of the weeds is assuredly a "Matthew" creation. I reproduce my arguments below:

<< The alleged explanation by Jesus:
Mt13:37-43 "He answered and said to them: "He who sows the good seed is the Son of Man [Jesus]. The field is the [known] world, the good seeds are the sons of the kingdom,
["kingdom": Mk = 21, Mt = 56, Lk = 45, Jn = 3. Note: all word searches based on NKJV]
` but the weeds are the sons of the wicked one.
["the wicked": Mk = 0, Mt = 9, Lk = 2, Jn = 0]
` The enemy who sowed them is the devil,
["devil": Mk = 0, Mt = 6, Lk = 6, Jn = 3]
` the harvest is the end of the age,
["end of the age": Mk = 0, Mt = 4, Lk = 0, Jn = 0]
` and the reapers are the angels.
["angels": Mk = 5, Mt = 13, Lk = 9, Jn = 2]
` Therefore as the weeds are gathered and burned in the fire, so it will be at the end of this age.
The Son of Man will send out His angels,
["son of man": Mk = 14, Mt = 32, Lk = 25, Jn = 12]
` and they will gather out of His kingdom all things that offend, and those who practice lawlessness,
["lawlessness": Mk = 0, Mt = 4, Lk = 0, Jn = 0]
` and will cast them into the furnace of fire.
["furnace of fire" Mk = 0, Mt = 2, Lk = 0, Jn = 0]
` There will be wailing and gnashing of teeth.
["wailing": Mk = 0, Mt = 3, Lk = 1, Jn = 0;
"gnashing of teeth" = Mk = 0, Mt = 6, Lk = 1, Jn = 0]
` Then the righteous
["righteous" or "righteousness: "Mk = 1, Mt = 15, Lk = 6, Jn = 5]
` will shine forth as the sun in the kingdom of their Father ...""
The wording of the explanation is quintessential "Matthew"! Compare this phraseology with the explanation of the sower and the soils parable in Mk4:13-20 quoted earlier in Section 5, and copied by "Matthew": Mt13:18-23. It is very different!

But what can be said about the parable itself?
First, let's examine its location within the gospel and compare it with the corresponding layout in GMark:
Mk4:1 Jesus teach from a boat > Mt13:1-2 Same
Mk4:2-9 Parable of the sower > Mt13:3-9 Same
Mk4:10-12 The secret of the Kingdom > Mt13:10-17 Same, plus Matthean and "Q" material
Mk4:13-20 Parable of the sower explained > Mt13:18-23 Same
Mk4:21 Parable of a lamp on a stand > (Moved to Mt5:15)
Mk4:22-23 Hidden to be disclosed > (Moved to Mt10:26)
Mk4:24 Measure you use ... > (Moved to Mt7:2)
Mk4:25 Given more ... > (Moved to Mt25:29)
Mk4:26-29 Parable of the growing seed > Mt13:24-30 Parable of the weeds
Mk4:30-32 Parable of the mustard seed > Mt13:31-32 Same
With the exception of Mk:4:21-29 material which "Matthew" dispatched in small pieces to other parts of his gospel, both layouts are the same. And Matthew's parable of the weeds is at the corresponding location of Mark's parable of the growing seed.

Now, let's search for any common element in the two parables:
The whole growing seed parable:
Mk4:26-29 "And He said, "The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come.""
The beginning of the weeds parable:
Mt4:24-26a "Another parable He put forth to them, saying: "The kingdom of heaven is like a man who sowed good seed in his field; but while men slept, his enemy came and sowed weeds among the wheat and went his way. But when the grain had sprouted and produced a crop,""
It seems that "Matthew" incorporated most of Mark's parable into his own. However, "Matthew", like "Luke" (who did not include it), had little use for Mark's parable of the growing seed, mainly because of its redundancy with the sower and the soils parable: Mk4:3-8 and Mt13:3-8.
Therefore, "Matthew" added on the "negative" element of the weeds, and from that point on, it becomes the focus of the expanded parable. And then "Matthew" could have been inspired by the following "Q" material:
Mt3:12 "His winnowing fork is in his hand, and he [Jesus] will clear his threshing floor, gathering his wheat into the barn and burning up the chaff with unquenchable fire."

Of course, "Matthew" was inclined to emphasize the punishments to be inflicted upon the "undesirables" at the second coming:
"furnace of fire", "hell fire" and "everlasting fire": Mk = 0, Mt = 7, Lk = 0, Jn = 0
"gnashing of teeth": Mk = 0, Mt = 6, Lk = 1, Jn = 0
"outer darkness": Mk = 0, Mt = 3, Lk = 1, Jn = 0
"wailing": Mk = 0, Mt = 3, Lk = 1, Jn = 0
"weeping": Mk = 0, Mt = 5, Lk = 2, Jn = 0
The Matthean parable of the net Mt13:47-50 is on of the same subject:
"Again, the kingdom of heaven is like a dragnet that was cast into the sea and gathered some of every kind, which, when it was full, they drew to shore; and they sat down and gathered the good into vessels, but threw the bad away.
So it will be at the end of the age. The angels will come forth, separate the wicked from among the just, and cast them into the furnace of fire. There will be wailing and gnashing of teeth."
And at the end of the "Q" parable of the great (here a wedding) banquet Mt22:12-13, "Matthew" added:
"So he said to him, 'Friend, how did you come in here without a wedding garment?' And he was speechless. Then the king said to the servants, 'Bind him hand and foot, take him away, and cast him into outer darkness; there will be weeping and gnashing of teeth.'
[there is no love of enemy here, not even friend!]"

In conclusion, "Matthew" used Mark's parable of the growing seed in order to expand on the treatment of the "undesirables" (the "weeds") at the second coming, a notion very dear for the author. >>

In conclusion, it is certain "Matthew" created the parable of the weeds from Mk4:26-29 and consequently "Thomas" got it from GMatthew.

Notes:
a) In "Parables and gospels", I argued extensively (and with backup evidence, I may add) that all the N.T. Jesus' parables are not his, and the "genre" was started by "Mark". That puts me in a small minority but I do not expect all my readers to follow me on that.
However, if agreed, then fourteen logions in GThomas would be "inspired" wholly or in part, by previously existing gospels (with "Q"):
8-9, 20-21, 33, 47, 57, 63-65, 76, 96, 107, 109
b) The parable, even the GThomas version, is highly eschatological: "The Kingdom of the Father is ... For on the day of the harvest the weeds will be plainly visible, and they will be pulled up and burned.", that is referring to a future apocalyptic "Day", believed by orthodox Christians. That goes against the notion of 'the Kingdom is already here' and does not seem to belong in GThomas. Then why would "Thomas" include this parable in his logions?
Likely to emphasize that the others (the weeds), not the members of his sect, are NOT the elects (and "Thomas" overlooked the eschatological implication of the parable!).

2.4 Logion 47:
CoGTh 47 "Jesus said, "It is impossible for a man to mount two horses or to stretch two bows [no parallel]. And it is impossible for a servant to serve two masters; otherwise he will honor the one and treat the other contemptuously [Mt6:24,Lk16:13]. No man drinks old wine and immediately desires to drink new wine [Lk5:39 only]. And new wine is not put into old wineskins, lest they burst; nor is old wine put into a new wineskin, lest it spoil it [Mk2:22,Mt9:17,Lk5:37-38]. An old patch is not sewn onto a new garment, because a tear would result [Mk2:21,Mt9:16,Lk5:36].""

Note: Let's do some visual comparison with the corresponding synoptic gospel sayings:
GTh "And it is impossible for a servant to serve two masters; otherwise he will honor the one and treat the other contemptuously"
"Q" Mt6:24/Lk16:13 "No one can serve two masters; for either he will hate the one and love the other, or else he will be loyal to the one and despise the other. You cannot serve God and mammon."
If we suppose, for an instant, the GThomas author used the canonical gospels:
"hate" and "despised" are replaced by a common "treat contemptuously"; then "love" and "be loyal" are substituted by one "honor". That allows the elimination of the "one" and "other" clauses. The overall result is excellent rewriting, allowing to express the same idea concisely!
Enough dreaming! But one can wonder why the "Q" author would have been so wordy, if a shorter and more concise version was known then.

GTh "And new wine is not put into old wineskins, lest they burst; nor is old wine put into a new wineskin, lest it spoil it"
Mk2:22 "And no one puts new wine into old wineskins; or else the new wine bursts the wineskins, the wine is spilled, and the wineskins are ruined. But new wine must be put into new wineskins."
Mt9:17 "Nor do they put new wine into old wineskins, or else the wineskins break, the wine is spilled, and the wineskins are ruined. But they put new wine into new wineskins, and both are preserved."
Lk5:37-38 "And no one puts new wine into old wineskins; or else the new wine will burst the wineskins and be spilled, and the wineskins will be ruined. But new wine must be put into new wineskins."
GThomas drops 'new wine to be put in new wineskins' but adds 'old wine not to be put in new wineskins'. Once again, we note the economy of words in GThomas. Also let's notice the rewriting from Markan material of both "Matthew" and "Luke". And then, none of the synoptic writers seems to have known about the GThomas version.

GTh "An old patch is not sewn onto a new garment, because a tear would result"
Mk2:21 "No one sews a piece of unshrunk cloth on an old garment; or else the new piece pulls away from the old, and the tear is made worse."
Mt9:16 "No one puts a piece of unshrunk cloth on an old garment; for the patch pulls away from the garment, and the tear is made worse."
Lk5:36 "... No one puts a piece from a new garment on an old one; otherwise the new makes a tear, and also the piece that was taken out of the new does not match the old."
The simplification in the GThomas version is again obvious.
"Luke" added "and also the piece that was taken out of the new does not match the old"
Here, it looks that the new piece is at fault, and the old garment is worthy of special consideration. In the next saying, it is clear that "Luke" liked the "old".

GTh "No man drinks old wine and immediately desires to drink new wine"
Lk5:39 "And no one, having drunk old wine, immediately desires new; for he says, 'The old is better.'"
Here, except for "for he says "the old is better"", GThomas and GLuke versions are very similar.
In "Parables and gospels", I made an argument which I reproduce now:

<< In 'Acts', "Luke" is a staunch supporter of Paul and consequently would be against any unPauline Christian teaching. See more about Luke's coloring in The great omission in Luke's gospel.
Luke's community was probably warned against new Christian teaching: keep the old one, it is better! Then "But new wine must be put into new wineskins" gets a different meaning: take that new unwanted stuff somewhere else! >>

Note: in Mk2:22 & Mt9:17, the new wine & wineskins are good, the old wineskins are bad and there is NO mention of old wine! "Luke" considerably distorted Mk2:22 through Lk5:39.

In conclusion, "No man drinks old wine and immediately desires to drink new wine", because of:
- Close similarity of wording between GThomas and GLuke's version
- Appearing only (relative to the N.T.) in GLuke and as an addition on Markan material
- Thoroughly explained by Luke's coloring & bias "The old is better"
originated from GLuke and consequently the GThomas version came later.

2.5 Logion 79:
CoGTh79 "A woman from the crowd said to him, "Blessed are the womb which bore you and the breasts which nourished you."
He said to her, "Blessed are those who have heard the word of the father and have truly kept it. For there will be days when you will say, 'Blessed are the womb which has not conceived and the breasts which have not given milk.'""
Let's compare it with two passages appearing only (among the N.T.) in GLuke:
Lk11:27-28 "And it happened, as He spoke these things, that a certain woman from the crowd raised her voice and said to Him, "Blessed is the womb [Mary's!] that bore You, and the breasts which nursed You!"
But He said, "More than that, blessed are those who hear the word of God and keep it!"
and
Lk23:27-29 "And a great multitude of the people followed Him, and women who also mourned and lamented. But Jesus, turning to them, said, "Daughters of Jerusalem, do not weep for Me, but weep for yourselves and for your children. For indeed the days are coming in which they will say, "Blessed are the barren, wombs that never bore, and breasts which never nursed!'"

Let's start by saying the two canonical passages are typically Lukan, that is featuring prominently woma(e)n. Luke's gospel has many of these, as I explained on this page. Here is a recapitulation:
Lk1:26-39 (Mary as a single emancipated woman), Lk2:48 (prominence of Mary over her husband), Lk7:44-47a (the loving woman who got her sins forgiven), Lk8:1b-3 (female followers supporting Jesus & disciples, above (& before!) what shows in Mk15:40b-41a), Lk10:39-42 (women should not be tied up to house work!), Lk16:18 (remarrying divorced women not at fault --modified from Mk10:12) & Lk21:16-18 (fear of execution secondary to losing hair (by shaving)).

Considering the overly pro-feminist (& pro-Mary) coloring of GLuke (way above what shows in the other Synoptics), it is most probable the two quoted Lukan passages are a creation of "Luke".
Also very suspect is the word "crowd(s)" which appears only here in GThomas, but is fairly common in the Synoptics, including GLuke (41 times). Why would "Thomas" have someone "from the crowd" when otherwise Jesus is dealing solely with an inner circle of disciples?
The only other exception is the "man" of logion 72, but here no "crowd" is mentioned.

With Lk23:27-29 obviously pointing at the destruction of Jerusalem (& most of its inhabitants) in 70C.E., why would "Thomas" use Lk23:29 in his logion?
Looking back at GTh 79, it is clear the words of God to be kept are about women not having babies, that is stay virgin/unmarried. That would fit very well with the "solitary" of GTh 16, 49 & 75, to whom the Kingdom is promised:
CoGTh 49 "Jesus said, "Blessed are the solitary and elect, for you will find the kingdom. ...""
CoGTh 75 "Jesus said, "Many are standing at the door, but it is the solitary who will enter the bridal chamber.""
And, in order to enter the Kingdom, man & woman are requested to be of the same neuter gender:
CoGTh 22 "... and when you make the male and the female one and the same, so that the male not be male nor the female female ..."
This is apparently drawn from the gospel of the Egyptians, as explained later on this page. And, from the same gospel, as witnessed by Clement of Alexandria in 'Miscellanies', procreation is not advisable:
3, 66, 2 "Salome says to Jesus, 'I did well, then, by not bearing [children]'."
and
3, 63, 2 "For Jesus really came 'to destroy the works of the female'."

Let's also note that among the five named disciples in GThomas, two are women, Mary (Magdalene) & Salome, a rather large ratio. And these two women are never said to have been married, in any canonical or uncanonical early Christian writings.

In conclusion, in view of the many "coincidences" with GLuke (on the wording) and GEgyptians (on the gist), it is most likely "Thomas" combined two typical Lukan passages (probably both created by "Luke"!) in order to justify (somewhat obscurely) that the women (as the men) in the sect must be "solitary" in order to "find the kingdom".

2.6 Logion 3:
GrGTh 3 "Jesus said, "If those who attract you say, 'See, the Kingdom is in the sky,' [heaven] then the birds of the sky will precede you. If they say to you, 'It is under the earth,'
[changed in CoGTh 3 as "'It is in the sea,'". "It is under the earth" alludes to Paradise, which in Hellenistic culture is a part of Hades, the underworld]
` then the fish of the sea will precede you. Rather, the Kingdom of God is inside of you, and it is outside of you. [Those who] become acquainted with [themselves] will find it; [and when you] become acquainted with yourselves, [you will understand that] it is you who are the sons of the living Father. But if you will not know yourselves, you dwell in poverty and it is you who are that poverty.""
First, let's notice the parallel with:
"This, says he [the Naassene], is the kingdom of heaven that reposes within us as a treasure, as leaven hid in the three measures of meal [Mt13:33-34, Lk13:21, GTh96]." (Hippolytus of Rome, The Refutation of All Heresies, V, III)

Now let's start the discussion by suggesting this logion might very well be an extrapolation from Lk17:21. Here is why:
First, I'll quote the preamble (the preceding verse):
Lk17:20 "Now when He was asked by the Pharisees when the kingdom of God would come, He answered them and said, "The kingdom of God does not come with observation;"
Now here is the aforementioned verse:
Lk17:21 "nor will they say, 'See here!' or 'See there!' For indeed, the kingdom of God is within you [but NO 'outside of you'!]."

2.7 Conclusion:
"Thomas" was addressing intellectual Christians, "solitary" (16, 49 & 75), not wealthy (54 & 110), "as innocent as doves" (39), who knew main orthodox (eschatological) beliefs like the future Kingdom (51, 113), second coming (37), apocalypse (11, 111) & resurrections (51). They had still a foot into Ebionism (high regards for James (12), Jesus the wise teacher/God messenger (13) (at best an adopted/honorary Son but not divine 44, 61), the Kingdom on earth, redeeming poverty (54) and observance of the Sabbath (37)) but were not Jews (14, 43 & 53).
And the gospel of Thomas is dependent on the canonical (& uncanonical, such as 'of the Egyptians'!) gospels (more to come!) and with some logions (and the prologue) calling for its composition/compiling done anytime from the very end of the 1st century to the beginning of the 2nd.
The probable dating of GThomas (the Greek version) is around 100-130C.E., at a time when the synoptic gospels (and others) were known, but still with limited acceptance (see "Parables and gospels", Section 7), when some sectarians groups still believed the true believer does not die, when Judas-Thomas, Mary, Matthew & Salome became prominent in Christian texts and when the 'Kingdom which did not come in time' required reinterpretations.

Let's note that about this time, according to Eusebius "The history of the Church", 3, 39, "Papias wrote five volumes entitled The Sayings of the Lord Explained" which had "some otherwise unknown parables and teachings of the Saviour ..."
Other 2nd century (or very late 1st) collections of sayings include the Didache ("Teaching of the apostles") (around 95), the Dialogue of the Saviour (120-180) and the Apocryphon of James (100-150), which also deals with secret books.

Note: the later shares some beliefs with GThomas:
"It is clear that the person for whom the tractate [Apocryphon of James] was written made a distinction between themselves and the larger Christian church. Probably they rejected the doctrine of the atonement; they certainly ignored the second coming of Christ and the general resurrection ... the kingdom of heaven, which they ... felt to be within themselves." (Francis Williams, 'The Nag Hammadi Library in English', Introduction)
Furthermore, the Apocryphon features new parables (as GThomas) and shows some Gnostic tendencies.

Notes:
A) The words "Christ" and "Lord" (for Jesus) never occur in GThomas, neither the beliefs of resurrection, Sacrifice, Satan and Jesus from &/or into heaven.
B) (True) Ebionites, at least in their beginning (around 70), shared the same beliefs as the "Nazarenes" (led by some of Jesus' eyewitnesses) and rejected the ones of Paul. See HJ-2b for "Nazarenes" and Ebionites
C) We know of another Ebionistic sect operating in Asia Minor during the last part of the first century, the one led by Cerinthus. The initial beliefs (which may have included early Gnosticism) appear to have been about the Holy Spirit (or a spiritual "Christ") entering a mortal Jesus at baptism and departing from him during (or before) the Passion, plus the Kingdom to come on earth.
D) Hegesippus, writing about the same time period (that is about 100-130), from Eusebius' 'The History of the Church', 3, 32:
"But when ... the generation of those privileged to listen [to the apostles] with their own ears ... had passed on, then godless error began to take shape, through the deceit of false teachers, ... by preaching the knowledge falsely so called."
E) The gospel of Thomas was likely written in Asia Minor, rather than (eastern) Syria, as commonly thought:
a) We know the Thomassan sect existed (as also another Ebionistic sect, the Cerinthians) where & when GJohn was being written (most probably in Ephesus).
b) Other "Thomassan" writings came much later and have little in common with GThomas, except 'Thomas'. The two next earliest ones are 'The infancy gospel of Thomas' (written 140-170) and 'The book of Thomas the contender' (written 150-225):
The former features "Thomas" only once and solely as the scribe. There, infant Jesus is described very unlike the wisdom (& non-divine) teacher: he is a brat, with divine extraordinary power, such as creating live birds from mud and (through words) killing someone & making others blind.
The later is fully Gnostic, with Thomas, the alleged author, being very close of Jesus the Savior and much in evidence.
Therefore, it is highly improbable these books are from the same sect and it is very misleading to put them in the same bag.

3. Counter arguments

Now, I will review and counter the main arguments in favor of early composition.

3.1 Different earlier redaction:
Argument:
In the case of the parallels, the GTh versions are not straight copies of the corresponding passages in the canonical gospels:
A) They are "primitive", simpler, shorter & "cleaner" because, as claimed, those sayings were "captured" very early on, before corruption & elaboration took place.
B) They are written differently, implying another origin.

What follows are partial answers, specific on the two aforementioned points. Then I'll respond to the argument, as a whole.

(Partial) answers:

A) I certainly do not deny, as a general rule, the GTh version of a parallel is streamlined relative to its counterpart(s).
However, I want to make a point here:
If someone rewrites some existing material, it is easy to improve on it, make it more concise and elegant. I know! I do that all the time on my own writing, sometimes on others too. Chances are, if not all the time, rewritten material will look better compared to something written (quickly) for the first time. A good example of that would be within logion 21, which exemplifies the insertion of cleanly composed canonical pieces into badly written material with no parallels (some typically "Thomassan"), resulting into a bizarre and confusing assemblage (parallels in italics):
CoGTh 21: "Mary said to Jesus, "Whom are Your disciples like?"
He said, "They are like children who have settled in a field which is not theirs. When the owners of the field come, they will say, 'Let us have back our field.' They (will) undress in their presence in order to let them have back their field and give it back to them.
[how strange and badly written is the last sentence, with an useless repeat! And no apparent connection with what follows!]
` Therefore I say to you, if the owner of a house knows that the thief is coming, he will begin his vigil before he comes and will not let him into his house of his domain to carry away his goods. You, then, be on your guard against the world. Arm yourselves with great strength lest the robbers [who would take your faith away!] find a way to come to you, for the difficulty which you expect will (surely) materialize.
Let there be among you a man of understanding.
[Jesus is answering Mary, a woman!]
` When the grain ripened, he came quickly with his sickle in his hand and reaped it.
[who is "he"? No obvious relation to what is preceding. And the whole sentence is begging for a context]
` Whoever has ears to hear, let him hear".
[why would Jesus say that to Mary then?]"

Parallels in the synoptic gospels:
Mt24:43 "But know this, that if the master of the house had known what hour the thief would come, he would have watched and not allowed his house to be broken into."
Mk4:26-29 (the parable of the growing seed) "And He said, "The kingdom of God is as if a man should scatter seed on the ground, and should sleep by night and rise by day, and the seed should sprout and grow, he himself does not know how. For the earth yields crops by itself: first the blade, then the head, after that the full grain in the head. But when the grain ripens, immediately he puts in the sickle, because the harvest has come
[this last sentence was probably isolated in Logion 21 in order to remove any eschatological content, very obvious within the context of the Markan parable].""
Mk4:9 "And He said to them, "He who has ears to hear, let him hear!""

Certainly, "Matthew", and more so "Luke", did rewrite Markan material.
As shown below, I give two examples, with the GMark, GLuke and GThomas versions. Let's see what can be learned from them.

The parable of the sower
Here, from Mark's gospel:
Mk4:3-8 "... a sower went out to sow. And it happened, as he sowed, that some seed fell by the wayside; and the birds of the air came and devoured it. Some fell on stony ground, where it did not have much earth; and immediately it sprang up because it had no depth of earth. But when the sun was up it was scorched, and because it had no root it withered away. And some seed fell among thorns; and the thorns grew up and choked it, and it yielded no crop. But other seed fell on good ground and yielded a crop that sprang up, increased and produced: some thirtyfold, some sixty, and some a hundred."

The much shorter & concise Luke's version:
Lk8:5-8 "A sower went out to sow his seed. And as he sowed, some fell by the wayside; and it was trampled down, and the birds of the air devoured it. Some fell on rock; and as soon as it sprang up, it withered away because it lacked moisture. And some fell among thorns, and the thorns sprang up with it and choked it. But others fell on good ground, sprang up, and yielded a crop a hundredfold."
Observations:
Superfluous details and wording were removed.

Let's examine the same parable from GThomas:
CoGTh 9 "... the sower went out, took a handful (of seeds), and scattered them. Some fell on the road; the birds came and gathered them up. Others fell on the rock, did not take root in the soil, and did not produce ears. And others fell on thorns; they choked the seed(s) and worms ate them. And others fell on the good soil and produced good fruit: it bore sixty per measure and a hundred and twenty per measure."
Observations:
Surprise! The GTh version is not shorter than Luke's one.
What to conclude? "Luke" also could abbreviate when rewriting.

The parable of the tenants
First, from Mark's gospel:
Mk12:1-9 "... A man planted a vineyard and set a hedge around it, dug a place for the wine vat and built a tower. And he leased it to tenants and went into a far country. Now at vintage-time he sent a servant to the tenants, that he might receive some of the fruit of the vineyard from the tenants. And they took him and beat him and sent him away empty-handed. Again he sent them another servant, and at him they threw stones, wounded him in the head, and sent him away shamefully treated. And again he sent another, and him they killed; and many others, beating some and killing some. Therefore still having one son, his beloved, he also sent him to them last, saying, 'They will respect my son.' But those tenants said among themselves, ' This is the heir. Come, let us kill him, and the inheritance will be ours.' So they took him and killed him and cast him out of the vineyard. Therefore what will the owner of the vineyard do? He will come and destroy the tenants, and give the vineyard to others."

Now, let's take a look at the parable in Luke's gospel:
Lk20:9-16 "... A certain man planted a vineyard, leased it to tenants, and went into a far country for a long time. Now at vintage-time he sent a servant to the tenants, that they might give him some of the fruit of the vineyard. But the tenants beat him and sent him away empty-handed. Again he sent another servant; and they beat him also, treated him shamefully, and sent him away empty-handed. And again he sent a third; and they wounded him also and cast him out. Then the owner of the vineyard said, 'What shall I do? I will send my beloved son. Probably they will respect him when they see him.' But when the tenants saw him, they reasoned among themselves, saying, 'This is the heir. Come, let us kill him, that the inheritance may be ours.' So they cast him out of the vineyard and killed him. Therefore what will the owner of the vineyard do to them? He will come and destroy those tenants and give the vineyard to others.'"
Observations:
Partly because some details in the front end got dropped, Luke's version is significantly shorter than Mark's one.
In GLuke, the owner sends servants, one by one, three times. They are beaten but not killed. "Mark" has one servant sent, then another one, then still another (this one gets killed), then many others (who are either beaten or killed). "Luke" simplified the whole scenario.

Now, let's see the still much shorter GThomas version:
CoGTh 65 "... There was a good man who owned a vineyard. He leased it to tenant farmers so that they might work it and he might collect the produce from them. He sent his servant so that the tenants might give him the produce of the vineyard. They seized his servant and beat him, all but killing him. The servant went back and told his master. The master said, 'Perhaps <they> did not recognize <him>.' He sent another servant. The tenants beat this one as well. Then the owner sent his son and said, 'Perhaps they will show respect to my son.' Because the tenants knew that it was he who was the heir to the vineyard, they seized him and killed him."
Observations:
a) Mark's details in the parable front are not present, as in GLuke (but they exist in Matthew's version). Also here, only two servants (who are merely beaten as in GLuke) get sent in succession, one less than in Luke's parable.
Did "Luke" set the trend?
I would say, likely so, and "Thomas" went one step further.
b) The ending (in italics) about the owner's revenge is missing. As explained in "Parables and gospels", Section 5, it alludes to God avenging his Son by destroying Jerusalem (70C.E.). Why would the author, as a second century man, remove any suggestion of it?
Jerusalem destruction long ago was not an issue any more, especially among Gentile Christians. GJohn does not mention it, unlike the earliest gospel, GMark.
c) However, without the owner's revenge on the tenants (as in the synoptic gospels), GThomas features "a good man" whose servants are beaten and his own son/heir killed; the murderers are unpunished!
Within Mark's gospel, the (complete) parable makes a lot of (historical/theological) sense. But the truncated 'without context' GThomas version looks immoral & leading nowhere, that is if not alluding to (some of) its N.T. meaning: an explanation about why the Son (=Jesus) got killed.
Consequently, it is likely Mark's version came first and Thomas' rendition was generated later (as we saw, mostly from GLuke).

Note: what about the changes from GrGTh to CoGTh?
For the proponents of an early GThomas, the difference in wording signifies independence of GThomas from the canonical gospels. But considering the liberties that the CoGTh so-called "translators" took on GrGTh, rewriting of gospel material would not be out of question!
Here are some other examples of rewriting from GrGTh to CoGTh (I emphasized the main differences):
a) GrGTh 2 "[Jesus said]: "Let him who seeks not cease until he finds, and when he finds he shall wonder; wondering he shall reign, and reigning shall rest.""
CoGTh 2 "Jesus said, "Let him who seeks continue seeking until he finds. When he finds, he will become troubled. When he becomes troubled, he will be astonished, and he will rule over the All.""
and
b) GrGTh 5 "Jesus said: "Recognize what is before your face and that which is hidden from the you will be revealed to you. For there is nothing hidden which shall not be made manifest, nor buried which shall not be raised.""
CoGTh 5 "Jesus said, "Recognize what is in your sight, and that which is hidden from you will become plain to you. For there is nothing hidden which will not become manifest.""
Let's notice here the deletion of "buried which shall not be raised", possibly NOT to remind the believers about unfulfilled resurrections. But the initial writing of these words (in the Greek version) was probably meant to provide new meanings: "buried" = "hidden", "raised" = "made manifest", away from the more literal (and orthodox!) ones.
The similar stratagem is used in what follows:
CoGTh 61 "Jesus said, "Two will rest on a bed: the one will die, and other will live ..."
which seems to be dealing with:
Lk17:34a "... there will be two in one bed: the one will be taken and the other will be left ..."
"Luke" was referring to 'the day of the Lord' (17:30) and alluding to the rapture ("be taken") of an elect, with the other one left behind, as rejected & exposed to God's wrath. But "Thomas", following his dual agenda (removing any event associated with the "Day" and "NOT dying" being the reward of the elect), attempted to replace the meaning of the key-words as such: "be taken" => "die" and "be left" => "live". Now, the second one becomes the elect!
c) GrGTh 30/77 "Jesus said: "Where there are [two, they are not] without God, and when there is one alone, [I say,] I am with him. Raise the stone, and there you will find me; cleave the wood, and there I am."
CoGTh 30 "Jesus said, "Where there are three gods, they are gods. Where there are two or one, I am with him."
CoGTh 77 "Jesus said, "It is I who am the light which is above them all. It is I who am the All. From Me did the All come forth, and unto Me did the All extend. Split a piece of wood, and I am there. Lift up the stone, and you will find Me there.""
Let's notice, in the Coptic version, the split in two parts and then the addition of material at the beginning of the second one.

Note: the following from GMatthew was likely a source of inspiration:
Mt18:20 "For where two or three are gathered together in My name, I am there in the midst of them."

B) I do not deny the phraseology in the GThomas parallels is not exactly the same as the one in the corresponding N.T. passages. But then, "Matthew", and more so "Luke", changed wording when dealing with Markan material. Also, there are few occurrences when "Luke" and "Matthew" agreed, word by word, on "Q" sayings, implying either one author, or both, made changes. As example, let's compare:
Mt5:3 "Blessed are the poor in spirit, for theirs is the kingdom of heaven."
with
Lk6:20 "Blessed are you poor, for yours is the kingdom of God."
See the differences. But now let's look also at GThomas:
CoGTh 54 "Blessed are the poor, for yours is the kingdom of heaven."
Remarks:
a) The CoGTh version seems to be entirely composed of elements drawn from Mt5:3 and Lk6:20, as a combination of both verses. Also, let's notice the grammatical awkwardness in GTh caused by "yours" instead of "theirs" (OR "the" (poor) instead of "you").
b) On my page HJ-2b, I demonstrated (with evidence!) that "Luke" changed "the" to "you" & "theirs" to "yours" (Jesus' disciples) in order not to restrict the Kingdom to the poor only. In other words, "Thomas" had to know GLuke for that logion!
c) The next logion also appears to be an amalgamation between Lk14:26 (red) and Mt10:38 (blue):
CoGTh 55 "Jesus said, "Whoever does not hate his father and his mother cannot become a disciple to me. And whoever does not hate his brothers and sisters and take up his cross in my way will not be worthy of me.""
The addition of "in my way" seems to imply a knowledge of GJohn (19:17), where, contrary to the Synoptics, Jesus is carrying his own cross.

General answer:
The 2nd century saw an explosion of Christian writings, many allegedly written by a disciple of Jesus (including secret books, as in the 'secret book of James'). Many of those, at first look, appeared "dated" and likely to be thought as forgeries by most Christians then.
But how to introduce a new "gospel" so late, and prevent an outright rejection?
By claiming it was written by a (posthumously famous) disciple (a "trusted eyewitness").
That's a first step.
But also by including logions dealing with known sayings/parables from older (ancient) gospels. However, a straight copy of those would be a give-away. Thence the rewriting into a more "pure" (basic) form, as allegedly "Thomas wrote down" himself!

In GThomas, the "recycled" canonical material is relatively of little importance. Most of the main "teaching" are coming from original Thomassan material in logions, sometimes blended with bits & pieces from the gospels, likely in order to instill some credibility into them (and/or to suggest a new meaning, away from the one in the N.T!).
For example, the first (and therefore critical) six logions in GrGTh all contain gospel material:

Logion 1: "He said to them: "Whoever hears these words shall never taste death.""
Jn8:52 "'If anyone keeps My word he shall never taste death.'"

Note: the GrGTh translation above may not be accurate and it is contested by Andrew Bernhard (and others) who renders it this way:
"And he said, "[Whoever finds the interpretat]ion of the[se] sayings will not taste [death].""
More details here

Logion 2: "[Jesus said]: "Let him who seeks not cease until he finds, and when he finds he shall wonder; wondering he shall reign, and reigning shall rest.""
Mt7:8 "he who seeks finds"

Notes:
A) Let's also consider:
CoGTh 92 "Jesus said, "Seek and you will find. Yet, what you asked Me about in former times and which I did not tell you then, now I do desire to tell, but you do not enquire after it.""
Note: "Seek and you will find" stands, but the disciples did not ask at the right time: that would explain why they did not find and therefore passed away!
B) Here is a twin of logion 2:
"He that seeks will not rest until he finds; and he that has found shall marvel; and he that has marvelled shall reign; and he that has reigned shall rest." Clement of Alexandria (late 2nd century), Stromateis V, XIV
This saying is most likely from the gospel of the Hebrews, because, earlier in the same book (Stromateis II, IX), Clement wrote:
"As also it stands written in the Gospel of the Hebrews: He that marvels shall reign, and he that has reigned shall rest."
From this uncanonical gospel, only a few fragments survive, as quoted by Clement, Origen and Jerome. It appears to be dependent on the synoptic gospels, as shown below:
"When the Lord ascended from the water, the whole fount of the Holy Spirit descended and rested upon him, and said to him, "My son, in all the prophets I was waiting for you, that you might come, and that I might rest in you. For you are my rest; and you are my firstborn son,
[of the Holy Spirit (and figuratively), not the Father. More acceptable for Jewish Christians!]
` who reigns forever.""
GHebrews, from Jerome, Commentary on Isaiah 11:2
Here is the corresponding passage in GMark:
Mk1:10-11 "And immediately, coming up from the water, He saw the heavens parting and the Spirit descending upon Him like a dove. Then a voice came from heaven, "You are My beloved Son, in whom I am well pleased.""
GHebrews was likely written in the early 2nd century:
In his 'The History of the Church', Eusebius reported Hegesippus (mid 2nd century) quoted the same gospel:
4, 22 "He [Hegesippus] also draw occasionally on the Gospel of the Hebrews"
and Papias (early 2nd century) probably knew about it:
3, 39 "Papias ... reproduces a story about a woman falsely accused before the Lord of many sins. This is to be found in the Gospel of the Hebrews"
What is there to conclude?
"Thomas" likely knew about GHebrews (as Papias & Hegesippus!), as he appears to have known also about the gospel of the Egyptians (as mentioned earlier).

Logion 3: "Jesus said, "If those who attract you say, 'See, the Kingdom is in the sky,' then the birds of the sky will precede you. If they say to you, 'It is under the earth,' then the fish of the sea will precede you. Rather, the Kingdom of God is inside of you, and it is outside of you. [Those who] become acquainted with [themselves] will find it; [and when you] become acquainted with yourselves, [you will understand that] it is you who are the sons of the living Father. But if you will not know yourselves, you dwell in poverty and it is you who are that poverty.""
Lk17:21 "... 'See here!' or 'See there!' For indeed, the kingdom of God is within you."

Logion 4: "Jesus said: "Let the old man who is full of days not hesitate to ask the child of seven days about the place of life; then he will live. For many that are first will be last, and last, first, and they will become a single one."
Mt19:30 "But many who are first will be last, and the last first [relative to the Kingdom!]."

Logion 5: "Jesus said: "Recognize what is before your face [on your own!] and that which is hidden from the you will be revealed to you. For there is nothing hidden which shall not be made manifest, nor buried which shall not be raised.""
Mt10:26/Lk12:2 " ...For there is nothing covered that will not be revealed, and hidden that will not be known."

Logion 6: "His disciples asked him and said to him, "How do you want us to fast? And how shall we pray? And how [shall we] give alms? And what kind of diet shall we follow?"
Jesus said, "Do not lie, and do not do what you hate, for all things are disclosed before truth. For there is nothing hidden which shall not be shown forth.""
Again Mt10:26/Lk12:2 and:
Mk4:22 "For there is nothing hidden which which will not be revealed, nor has anything been kept secret but that it should come to light."

Remark: the answer of Jesus does not match the questions! However the logical answer appears in:
CoGTh 14 "Jesus said to them, "If you fast, you will give rise to sin for yourselves; and if you pray, you will be condemned; and if you give alms, you will do harm to your spirits. When you go into any land and walk about in the districts, if they receive you, eat what they will set before you, and heal the sick among them. For what goes into your mouth will not defile you, but that which issues from your mouth - it is that which will defile you.""
Then again, a part of the logion contains gospel material:
Lk10:8-9 "... and they receive you, eat such things as are set before you. And heal the sick there ..."
Mk7:15 "There is nothing that enters a man from outside which can defile him; but the things which come out of him, those are the things that defile a man."

We have to wonder about the mix-up between the questions of logion 6 and the alleged answers by Jesus.
If the author did not care about order here, why would any sequence from the canonical gospels be kept?
That brings us to the subject of randomness ...

3.2 Random locations:
Argument:
In the case of the parallels, the GTh versions are not bunched together (as per gospel) and appear in no particular order (as compared with the one of their counterparts in a canonical gospel). Why?
The defenders of an early GTh reply: parallels in GThomas were not drawn from the gospels.

Answer:
As for "straight copies", any order, as in the gospels, would have been a give-away.
And our author pushed the principle of randomness to some extent:
The two parables, "new cloth on an old coat" and "new wine in old wineskins", appear in that order, and one right after the other, in all the three synoptic gospels (Mk2:21-22, Mt9:16-17 & Lk5:36-38).
In GThomas 47, these two parables are also contiguous, but in reverse order!

However, the parable of the tenants (logion 65) is followed by "the stone which the builders have rejected" saying (logion 66) as in all the synoptic gospels.
Furthermore, we have the two consecutive logions:
CoGTh 93 "<Jesus said,> "Do not give what is holy to dogs, lest they throw them on the dung-heap. Do not throw the pearls to swine, lest they grind it [to bits].""
CoGTh 94 "Jesus [said], "He who seeks will find, and [he who knocks] will be let in.""
which have parallels in GMatthew in also consecutive verses:
Mt7:6 "Do not give what is holy to the dogs; nor cast your pearls before swine, lest they trample them under their feet, and turn and tear you in pieces."
Mt7:7 "Ask, and it will be given to you; seek, and you will find; knock, and it will be opened to you."

In the 26 close parallels, we have only two cases of pair of contiguous gospel sayings. If the 26 corresponding logions were taken randomly from a "pile" (and simplifying GThomas as consisting of only 26 logions), what are the odds to have the exact match in GThomas as two doublets in the proper sequence?
One chance out of 494
(see following appendix for calculation)
But GThomas has 113 logions (the last (very Gnostic) one excluded).
Then if the 113 logions came from a 'sayings' source (and none extracted from the gospels), what are the odds to have two sets as logions 65-66 and logions 93-94, each one matching a pair of consecutive N.T. sayings (and in the right order)?
About one chance out of 12,000
(see following appendix for calculation)
In gambling terms, the author hit the jackpot first time when normally he would need an average of twelve thousand tries.
That's remarkable, mind-boggling.
Or maybe "Thomas" was a bit careless.
So much for "randomness", as the "proof" of GThomas independence from the gospels!
And one more reason for the parable of the tenants to come from the synoptic gospels!

Appendix:
Calculation of odds: a bit of math!

A) Let's start with 2 items (a & b).
Possible combinations:
ab, ba (total: 2)
For one doublet (i.e. 'ba'), we have 1 occurrence (shown in red), out of the 2 combinations:
That's 1 chance out of 2 possible combinations, one chance out of 2. Also, let's notice that for 2 items, we have 2 combinations.

B) Let's proceed with 3 items (a, b & c).
Possible combinations:
abc, acb, bac, bca, cab, cba (total: 6)
For one doublet (i.e. 'ca'), we have 2 occurrences (shown in red), out of the 6 combinations:
That's 2 chances out of 6 possible combinations, or one chance out of 3. Also, let's notice that for 3 items, we have 6 combinations.

C) Now, let's consider 4 items (a, b, c & d).
Possible combinations:
abcd, abdc, acbd, acdb, adbc, adcb
bcda, bdca, bdac, bacd, bcad, badc
cdab, cabd, cbda, cdba, cadb, cbad
dabc, dcab, dacb, dbac, dbca, dcba (total 24)
For one doublet (i.e. 'bd'), we have 6 occurrences (shown in red), out of the 24 possible combinations:
That's 6 chances out of 24, or one chance out of 4. Also, let's notice that for 4 items, we have 24 combinations.

D) The equation is evident:
With N being the number of items, for one doublet we have one chance out of N.
For 20 items, one chance out of 20, for 113 of them, one chance out of 113
What is the relationship between possible combinations and items?
Let's recapitulate: 2 for 2, then 6 for 3, then 24 for 4.
With M being the number of combinations, the equation is M = N X (N - 1) X (N - 2) as long as (N - x) is more than 1.
For 2 items, M = 2
For 3 items, M = 3 X 2 = 6
For 4 items, M = 4 X 3 X 2 = 24
For 5 items, M = 5 X 4 X 3 X 2 = 120

E) What about the odds of satisfying two doublets?
a) In the example with 4 items, we have only 6 combinations which include the doublet 'bd':
abdc, acbd, bdca, bdac, cabd & cbda
Other possible doublets (without 'b' or 'd') are 'ac' & 'ca'. Out of the 6 combinations, only 2 of them (shown in blue) include 'ca' (and 2 others 'ac'); which means that the chances of satisfying two doublets (each one with different items) are 2 out of the 24 possible combinations, or one chance out of 12.
b) If we have 5 items (a, b, c, d & e), only the following combinations include one particular doublet, such as 'bd':
aebdc, abdec, abdce, aecbd, acebd, acbde
bdeca, bdcea, bdcae, bdeac, bdaec, bdace
ceabd, caebd, cabde, cebda, cbdea, cbdae
eabdc, ebdac, eacbd, ecabd, ebdca, ecbda (total 24)
That's 24 valid combinations for (as calculated earlier) 120 possible ones (and one chance out of N = 5). How many of those satisfy also another doublet (without items 'b' & 'd'), like 'ca'?
Only 6 (as shown in blue), out of 120. That's one chance out of 20.
c) Let's recapitulate:
To satisfy two doublets, for all possible combinations based on
- 4 items, one chance out of 12
- 5 items, one chance out of 20
The equation is then: X = N X (N - 1)
For 26 items, we have one chance out of (26 X 19 =) 494
For 113 items, we have one chance out of (113 X 112 =) 12,656

Probably the best entry point for the GThomas debate, learned opinions and additional info is:
http://www.earlychristianwritings.com/thomas.html

Read my analysis about the Q source right here