Questions about the Bible
Proving the Gospel using a Historical Document Analysis
Internal
Criteria 1
Was the author in a position to know what he was talking about with the four Gospels?  Well, yes, all four of them were.  All four authors had a personal relationship with Jesus Christ for some time.  When Matthew and Luke give their accounts of the birth of Jesus, it is obvious that they were not there, however, in traveling with Jesus as they did, there is a very good possibility that they would have come in contact with Mary and Joseph at least once, in order that they might learn of Jesus' birth.  There is no doubt that Jesus also knew the story of His birth and could have very easily articulated it to His disciples. 
Criteria 2
The Gospels are packed with specific and irrelevant details.  We will look at one passage to illustrate this fact: 

Early on the first day of the week, while it was still dark (who cares if it was still dark) Mary Magdalene went to the tomb and saw that the stone had been removed from the enterance.  So she came running to Simon Peter and the other disciple (who we know is John, the author), the one Jesus loved (Jesus loved everyone) and said, "They have taken the Lord out of the tomb and we don't know where they have taken him!"
So both Peter and the other disciple started for the tomb, but the other disciple outran Peter and reached the tomb first (
who cares who made it to the tomb first). He bent over and looked in at the strips of linen lying there but did not go in (more irrelevant information) Then Simon Peter, who was behind him, arrived and went into the tomb.  He saw the strips of linen lying there, as well as the burial cloth that had been around jesus' head.  The cloth was folded up, separate from the linen (so?) Finally the other disciple, who reached the tomb first, also went inside (the order that they went inside the tomb is irrelevant).  He saw and believed. (They still did not understand from Scripture that Jesus had to rise from the dead). - John 20:1-9

This passage is filled with irrelevant information that only someone who had a first hand account could have known, and they are details that are so percise that someone could not easily make them up, at least with the wrtting style at the point in time this was written. 
Criteria 3
Incriminating details in the Gospels?  Sure there are a bunch. Take a look at that last passage, and notice who the first person to go and to Jesus' tomb and then go back to find the disciples.  Mary Magdalene, a WOMAN.  For Pete's sake, a women was not a credible source for anything at this point in time.  For an author to include this would be a completely absurd, unless of course if the Gospel of John was not made up, then they would have no reason to not include details of this nature. 
Criteria 4
There are four Gospels, each wrtten by different people, they all contain the same details and for the most part they are all in the same order.  Names do not change inside any of the Gospels, with the exeption of cases like Simon Peter who became Peter and Saul who became Paul.  The chronology of the Gospels remains fairly accurate between all four accounts.  This four accounts each give a large amount to the credibility of the Gospels as a whole.
Criteria 5
Legendary accretion in the Gospels?  Its possible, if there was more time between the writting of the Gospels and the Resurrection of Jesus Christ.  Matthew was written within 10 to 20 years after the Resurrection, Mark and Luke within 10 years of Matthew being written, and John within 20 years of Mark and Luke.  So all four Gospels were written within  50 years of the Resurrection of Jesus.
External
Criteria 1
Picture yourself in Jesus' day, around the time, say 20, 30 AD.  The predominent religion of Israel at that point in time was Judaism.  To go against, or to write against that was often followed with harsh criticism, or more commonly worse.  Often people were stoned to death, or killed in various other manners for actions that went against the church of that day.  The actions of the four Gospel writers did just that.  Not only did they have motive to not write the books, they were putting their life on the line in writting what they did.  Logically then we can determine that the Gospel writters had nothing to gain from writing these books, and they had everything to lose.
Criteria 2
Do other sources confirm what was written?  For starters, at least four independant authors recorded what they observed and learned.  The accounts of all four are very similar, with only slight differences.  This can lead one to believe that there was a great deal of accuracy between the four authors.  We can also confirm, through other writtings, many of the details that were described through the Gospels, with other historical documents, such as the ruthless actions of Pilate.
Criteria 3
For a period of time, it was archeologists lack of information that lead them to be able to disprove many part of the Bible, the Gospels included.  But now as our knowledge of the past increases, we find that there is more and more example turning up in archeological digs that prove the accuracy of the Bible and the Gospels.  One such example is the proof of the use of crusifixion as a means of killing those found guilty.  For a period of time, the only information we had about crusifixion was in writtings and pictures. 
Criteria 4
Many, many writers have written against the Gospels, in attempts to disprove what has been writtent there.  One example of such could be The Case Against Christianity.  However,. the defences offered by such books fall short of being able to do anything but show the ignorance of the author.  Most of the points that are made, either come about through evolution, or simple ignorance of history.  Evolution is a set therorits which are more often than not shot down within a short time that each new therory is thought of.