I've studied the world's major religions for years but it was not until a couple days ago that the following question came to me. I was watching a documentary on H2 (previously History International) called "Secrets of the Koran" (1 hour 30 minutes), and aside from the fact that the illegible map they used and everything else was bathed in red hues making it difficult to see what was going on, and that it was a typical "hagiographic" documentary about Islam (I have yet to see a Western documentary on Islam that even has one person interviewed who is even willing to raise a single question regarding the orthodox Sunni position on the history of Islam and Muhammad), a couple of facts, or statements, piqued my curiosity. I'm sure I had heard all of these facts before but they never came together until the weekend.
For sake of argument, let us assume that the orthodox Sunni account is correct in all these facts (I have no objection to any of them). What is interesting is the possible interpretation that can be drawn.
1. Muhammad was illiterate. This is said to be one of the important facts concerning why the Quran is genuine revelation. It is argued that since Muhammad was illiterate he could not have copied stuff from other holy texts (or other texts in general, like all the medical knowledge from the Quran that was known by pagan doctors like Galen of Pergamon five centuries earlier). Okay, most people were illiterate back then (an even today) so it is no stretch of the imagination to suppose Muhammad was illiterate as well.
2. The text of the Quran was memorised perfectly by companions of Muhammad for around 30 years before it was compiled by Caliph Uthman (although the diacritical marks were added 50 or more years later, without which the Uthmanic text can be pronounced in a number of different ways, leading to different interpretations, because written Arabic, like Hebrew, has no vowels). Again, there's no problem here. People living in an oral culture are trained from a very early age to develop fantastic memories. Bramins in Kerala are trained to memorise the four Vedas twice, once as it is and once as one continuous sound. Poets memorised very long epic texts like the Illiad and Mahabarata.
Can you see what my question is yet? I'll give you a hint: Muhammad was a merchant before he became a prophet at age 40.
If we take as fact that Muhammad was an illiterate merchant and that people from his village (and certainly Muhammad himself, since he was the first person to memorise the text of the Quran!) were capable of fantastic feats of memory, how is fact number one proof again that the Quran is divine revelation? The Israelites, Greeks, and Romans travelled all over the western regions of the Arabian peninsula, and the Sassanid Persians controlled the east and south (present Yemen and Oman). Both cultures had advanced technological and scientific writings, and well developed and cosmopolitan religions (there were missionaries from many Christian and Gnostic sects as well as Jewish merchants and members of the Persian mystery religions). Arabia was bathed with knowledge, secular and religious. Muhammad would have encountered Jewish, Christian, Gnostic, Persian, and Greek religious and scientific ideas when he made his rounds trading salt or dates or whatever it was he made a living from. Since he, and many people, had very good memories, there is no reason to assume he could not have heard stories from Jews, Christians, Zoroastrians, and others and memorised them. Saying Muhammad was illiterate in no way demonstrates that the Quran is divine revelation.
If we look at the Quran we find a number of things: Greek medical and geological knowledge, Gnostic interpretations of Jesus (like that Jesus was not crucified, a look-alike was), "Reader's Digest" versions of stories about Abraham and other Jewish figures. It could be said that God revealed all this to Muhammad, I have no problem with that, but it means that evidence has to be brought up to cast doubt on the cryptomnesia hypothesis that Muhammad had heard all this stuff while he was a merchant, forgot about it, and then subconsciously reworked it together to form the Quran. Both scenarios are possible, and right now both have precisely the same amount of evidence.
As of now I do not have an answer to this question, nor do I forsee that I will at any time in the future. The most I can hope for is that more people will take this question to heart and ponder over it.