After the study of comparative religions, over a number of years, I have discovered that Judaism, Christianity and Islam are not only the three children of a common patriarch sharing a common heritage, but they all have a common belief in the existence of Revealed Scriptures. The preaching and practices of these religions are based upon the guidance and the norms that are dictated in their Holy Books. There is a heavenly thread that binds these books of Divine Instructions. The 'Great Instructor' being the ONE, I find conceptual similarities in quite a few instances between the  norms dictated in the Old Testament, the New Testament and the  Qur'an (Koran). Upon a closer study, one can easily notice that there happens to be a continuity of the Divine Instructions, linking one to another.  Furthermore, these dictated fundamental Instructions are of  a progressive nature. 

The bridges can be built between the three children of  Abraham - "People of The Books" - if every son and daughter of  the great patriarch reads with a wider perspective, the books of the other family members and respect their prophets. Reading of all the three scriptures, specially what the great prophets  themselves had originally spoken or revealed, is bound to develop  an attitude whose outlook would be panoramic enough to sweep the  conceptual harmony between them. In the Qur'an, Jews and Christians  are addressed as the "People of the Books". However, from an overall perspective, Muslims too are People of the Book, since Islam is  also founded upon a Revealed Book - the Qur'an. 

The development of a newly born child's ability to successfully communicate his thoughts by speech is not possible within a short period of few years. The parents and teachers have to impart that skill gradually and through various progressive stages. The process demands years of hard labor and patience. The warmhearted compassionate parental love turns this endless and staggering task into a greatly overwhelming pleasure. The results of all these enormous efforts are immeasurably rewarding.  

Similarly, the development of mankind's ability to efficaciously comprehend its Creator, Sustainer and Benefactor is not possible within a short period of few generations. After the comprehension or acquiring of the belief comes the stage of acknowledgment or the confession of faith. The God in its infinite Mercy has been sending various Messengers to impart that spiritual knowledge gradually. The progressive step by step Divine Instructions (the Cardinal Messages), as recounted below. 

LISTENING: Prophet Moses (a.s.) - the stately consolidator of  the early Semite tribes, taught his people the credo of 'Shema';  "Hear, O Israel; the Lord our God, the Lord is one". Deut. 6:4.  Moses, who came nearly six and half centuries after Abraham, instructed the Children of Israel to repeat and hear, this  Primary Fundamental Message for the 'Unity of God' (Tawhid),  in their ritual prayers, thrice a day. The process of listening  over and over again was like a primary school education. 

UNDERSTANDING: Prophet Jesus (a.s.) - the righteous Messiah, advanced the Primary Message of his predecessors and asked his followers to Know, Understand and Comprehend what they have been hearing for the last twelve centuries. Prophet Jesus said; "And this is eternal life, that they may know Thee the only true God and Jesus Christ whom Thou hast sent. I glorified Thee on earth, having accomplished the work which Thou hast given me to do." John 17 : 3-4.  The discernment was of THEE -  the Sender, as the "Only True God", and of Jesus - the one who was Sent, as the Sender's obedient Messenger. Jesus came to glorify the One and Only True God who had sent him. The recognition was of two entities having distinctly dissimilar status. Knowing this Truth was a kind  of Secondary education in the history of the mankind's Spiritual knowledge. 

TESTIFYING: Prophet Muhammad (a.s.) - the seal of Prophets, gave the final instruction to his Ummah (Brotherhood). Nearly seven centuries after his predecessor, prophet Muhammad asked his followers to Affirm and Confess the Eternal Message with faith. A Muslim is supposed to testify with faith; "I perceive that there is no god except 'The God' and I perceive that Muhammad is the Messenger of 'The God'." This definitive confession for the Oneness  of 'The God' combined with the absolute negation of its divisibility, was the Final Message. One may call it the Final Graduation. Hence, the need for a new Scripture and a succeeding  Educator to enlighten further, was not necessary. A believing  Muslim submits his/her whole self to Allah (The God). A Muslim acknowledges that  whatever has been Divinely Revealed or Inspired to the earlier prophets, in its untainted form, came from the One and Only Almighty Allah. He Himself had placed therein, the Divine Light and Guidance, for mankind. 

        Nor be like those who say, "We hear," but listen not: 
        For the worst of beasts in the sight of Allah are the 
        deaf and dumb, - those who understand not.  
                                Sura Al-Anfal (8): 21/22 

        ...Each one believeth in Allah and His angels and His 
        Scriptures and His messengers - we make no distinction 
        between any of His messengers...  Sura Baqarah (2): 185