An interesting historical observation by Quiet George:
[Recent] historical research…seems to suggest that Muhammad was not originally from Mecca at all (hence the stunningly few references to it the Qu’ran), but from much farther north, near the border of the Roman Empire (hence why the Qu’ran‘s only reference to a different political power was Rome). Interestingly, the ‘tribe’ that Muhammad claims to be a part of, the Quraysh, seems not really to be a real Arab tribe, nor is it is even an Arabic word, but rather a derivative of the Syriac word for “federation”, meaning the various northern Arab tribes allied to Rome. The Qu’ran itself tells a multitude of stories, all of which can be found in the OT and NT. This suggests that Muhammad thought of himself as saying nothing more than was in the Bible; and this would make sense if he was a Northern Arab, since the Northern Arabs were in fact Nestorian Christians. This also helps to explain the general tenor of the Qu’ran, which is not “I have a new religion,” but, “live righteous lives because the end is coming soon.”
Muhammad seems to be criticizing hypocrisy within his own religion (some backwater Nestorianism) and not founding a new one. He therefore travels to a new town to start a “community of true believers.” In the immediate aftermath of the Arab invasions, it is interesting to note that there is no mention of an “Islamic” religion, and that Arabs thought of themselves as fellow “believers” with Christians and Jews. No contemporary Christian source makes any reference to a new prophecy which they believed or that the Arabs believed a different religion. Whenever Hagarism is mentioned, it is mentioned only as a heresy of Christianity. Early Arab warlords who conquered the Middle East are described by contemporary historians as praying in churches and dining with priests and patriarchs. More interestingly, the coinage of the period shows Arab leaders crowned with crosses and holding symbols of Christianity in their hands. As the theory goes, later on, Arab religious leaders after the conquest feared that the “community of true believers,” that is, the Arabs, were intermingling with the hypocritical Christians of Egypt and Palestine and wished to divide their form of rigorism from common Christianity or Judaism. In time, this led to stories of Muhammad (the Hadith, compiled over a century after the conquests), describing him as teaching something distinctive from Christianity and Judaism; and it is during this time period that the term “Muslim” and “Islam” first come into use.
Caveat: Everything I said above is merely what I remember from a certain “Islam” phase I had a year or two ago (and what I learned recently from a conversation with my dad, who has also been reading up on this kind of thing), so I might be conflating a few different theories (since I read a few books on the topic on Islam’s origin). I think the theory I put forward is more or less the same as Thomas Holland’s, if I remember correctly. As for the coinage thing, I remember seeing a photo of it in one of these books. As for the anti-Christian verses, the theory goes that whenever Muhammad speaks of “Christians” and “Jews” he means it in the same way that John does when he mentions “the Jews” that is, false Christians or false Jews; the unbelievers or hypocrites. As for denying that Jesus was the Son of God, this is not too far of a jump from Nestorianism, and it seems that some groups on the fringes of Nestorian territory and influenced by Jewish populations may have had a somewhat Ebionite opinion on Jesus, that he was inspired and was the Messiah, but was not, technically speaking, God.
Heresy sure can be deadly, huh?