Personally I think Islam was seen (perhaps even made to be) first as yet another unorthodox, "heretical" (from the perspective of Eastern Roman authorities) Christian sect. That might even explain why Middle Eastern Christians didn't resist the Arab conquest much. They were oppressed by "official" orthodox Roman Christianity, but in many places a majority of them followed unorthodox Christian beliefs and often felt very oppressed for their beliefs, and they would now only switch allegiance to a different kind of broadly the same faith. Thus Mohammedism must've been viewed as a sort of Arianism, not as a distinct religion, and while he was alive or soon after he died Muhammad would've been something like a great leader and teacher proposing a new interpretation of the scriptures and a new "Arabized" version that was mishmash of Christian and Jewish customs and beliefs with an Arabic spice. Muslims never want to think much about it, but why were hadiths and the Sunnah compiled and turned into a significant part of the Islamic doctrine only MANY generations after Muhammad died, I mean, centuries after he supposedly built Islam entirely as it is during his life? Strange. I think the role of Muhammad as a central figure distinguishing Islam from Christianity appeared gradually through many decades or even centuries after his death. He was at first something like Smith for Mormonism.