The Gospel of John is at odds with the synoptic gospels. But even more compelling is that it rewrites human history and God’s relationship with man. In this way it is comparable to L.Ron Hubbard’s efforts to communicate the truth, Joseph Smith’s Book of Mormon, The Quran, and of course Genesis. It is the definitive starting point and basis for a religion. 
The gospel of John is at odds with the other gospels in how it portrays Jesus. Just reading the synoptics leaves you with a major prophet who hints at a special relationship with God. Islam recognizes this very well and accepts these stories of Jesus with some reservations (many believe Jesus was never crucified). But the gospel that they must reject is John, with many claims of divinity of Jesus. Plain and simple it takes the Gospel of John to make Jesus God. 
The books I mention above are sacred books for their religions. The followers often take their words to be an actual account of how the world and humanity came into existence. The Gospel of John begins with: “ In the beginning was the Word, and the Word was with God, and the Word was God.  He was with God in the beginning.  Through him all things were made; without him nothing was made that has been made. In him was life,and that life was the light of all mankind. The light shines in the darkness, and the darkness has not overcome^{[** you do not have permission to see this link **]} it. ” (NIV, Bible Gateway copy and paste while taking out those annoying verse numbers.) The entire history of creation has just been rewritten! This is not a suggestion that maybe Jesus was there all along, or that maybe Jesus watched the whole thing. This is a claim that God created through Jesus. This was unknown to the Jewish religion. Certainly Genesis made no mention of it. No prophet made mention of creation in this way, or of God even having one son that sat at his right side.
But what of the synoptics? Are they not the foundation for this new religion? I would say that they are an explanation of Jesus Christ and what he meant to his followers. They were one of many sects of Judaism which seemed to spread almost because of their resistance to bowing to the Son of God (Caesar), but that is something for another thread. 
The founding book of Christianity, therefore, is the gospel of John.
I will say though that this leaves me with a very tough question. The writers of the Book of Mormon, The Quran, Hubbard’s SciFi, and Genesis had much to gain with their revelations being written down as sacred texts. I wonder what the writer of the Gospel of John felt he had to gain?