Boy, that says it all.
Whether you believe in the existence of a deity or not, those who claim some book or other is inspired by or dictated by "he who cannot be seen", to borrow a J. K. Rowling conceit, have a large mountain to climb.
Biblical reseach has cast a serious doubt on whether some itinerant preacher in the Middle East, named Jesus, actually existed. The sources outside the Gospels are scant and the stories of the Gospel, written well after the death of this person, are too cutely arranged to make it appear that Jesus was the prophesied Messiah of the Jews. Too convenient to be merely coincidental.
The existence of Abraham, the father of the three Abrahamic religions is also not a matter of clear existential record. His existence is only found in proseltysing religious texts.
Finally, we have no less than a respected (soon to be reviled and perhaps condemned to death) Islamic scholar questioning the existence of Mohammed. He doesn't deny the existence of a deity, but his comment that "God doesn't write books" should be enshrined as the most notable quote of 2008.