Did Muhammad Exist? (Why That Question Is Hard to Answer)is any plausibility to the claim that there was no Muhammad, that he was invented by early Arabic military leaders to give a name to a text they cobbled together to inspire their soldiers and build a new civilization on? In other words, that he was invented in exactly the same way and for exactly the same reasons as the militaristic Jews invented Moses?
Spencer’s theory, for example, is that one can adduce internal and external linguistic evidence that the Quran is just a redaction of a pre-Islamic Syriac holy text (or texts) actually foundational to a Middle Eastern sect of Jewish Christians centuries before (this is also now argued by others, including Karl Ohlig, discussed below). And certainly, Islam is actually just a sect of Christianity (as much as Muslims—and Christians—would be horrified to admit it; Islam is in many ways just the first Mormonism), and clearly descended from the obscure but more original Torah observant wing of Christianity (hence their common requirement of circumcision, their common prohibition against consuming pork, and other details). Other scholars have proposed similar theories (as even acknowledged on Wikipedia), that the Quran is not really all that original, and Spencer is drawing on their previous work—indeed, more experts continue to make similar arguments. But crucially, most others who do, didn’t and don’t conclude this entails Muhammad didn’t exist.
Indeed, Spencer’s theory is compatible with historicity, since Muhammad might have been the very one to do that adaptation and pass it off as an original work—adding, perhaps, details about himself and his use and gradual development of the treatise during his life. Although Spencer claims, “we can glean nothing” from the Quran “about Muhammad’s biography,” because such details are too vague, and it is not “even certain, on the basis of the Qur’anic text alone, that these passages refer to Muhammad, or did so originally”
Likewise, while even adding a historical Muhammad back into Spencer’s theory would still be damning to fundamentalism (which requires the text to have been revealed by an angel of God and not a half-assed plagiarism of heretical Christian scriptures)
https://www.richardcarrier.info/archives/8574