I know what you are all thinking. "Please not another 'What if Muhammad became a Christian?' thread!" I am aware that this has been discussed many times, I have read most of the threads (and I know Turtledove's "Agent of Byzantium" as well).
Here, however, I would like to discuss a question that has not, to my knowledge, been discussed in much detail in these other threads: how, exactly, could it happen that Muhammad converts to Christianity? What could be the exact circumstances behind this?
We know that, by Muhammad's lifetime, Jewish and Christian communities had existed in Arabia for hundreds of years and traditional Arabic paganism was apparently already in retreat in the face of this spread of monotheism. We have very little information on these Arab Jewish and Christian communities, but it seems that larger Christian communities existed in the Yemen, in eastern Arabia, and in the far northern parts of Arabia bordering Syria and Mesopotamia. By contrast, there seem to have been no larger Christian communities in the Hijaz (the western side of Arabia where Muhammad lived), although individual Christians apparently existed there and the religion was not unknown (Fred M. Donner, Muhammad and the Belivers: At the Origins of Islam, Harvard University Press 2010).
There was also the Christian kingdom of Axum in Ethiopia, which had trade contacts with western Arabia across the Red Sea. According to Islamic tradition, some of the early followers of Muhammad fled the persecution of the Quraysh in Mecca around 613-615 CE and found refuge in Axum.
With this in mind, here are my thoughts:
- I think that the Arabic Christian communities in Yemen and on the fringes of Syria and Mesopotamia are too far away from Mecca to plausibly posit that Muahmmad would make contact with them or travel there, at least at an early stage of his life when he is still in Mecca with almost no followers and no means at his disposal. So this is not a good PoD.
- More realistic would be a PoD where Muhammad goes to Axum. After the death of his wife Khadija and his uncle Abu Talib, Muhammad's position in Mecca deteriorated rapidly and he had to leave. IOTL, he received an invitation from a group of men from Yathrib (Medina) to be an arbiter in their affairs. What if he had not received this invitation? In my mind, the most plausible thing for Muhammad to do would then have been to seek refuge in Axum, where some of his followers had gone before. Once in Axum, he might seek the support of the king by emphasising the common ground between the Christian faith and his own movement, maybe going as far as to declare himself a Christian and officially convert to Christianity. It is then conceivable that the Axumites could sponsor an expedition of Muhammad and his Arabic exiles to take over Mecca, a town where they had tried to set up a friendly ruler before (the Axumite viceroy of Yemen, Abraha, had attacked Mecca around 570 CE).
- Another PoD would be Muhammad going to Yathrib (Medina), as he did IOTL, but then getting killed in one of the early battles against the Meccans, such as the Battle of Uhud in 625 CE or the Battle of the Trench in 627 CE. In this case, Islam as a new, separate religion would be butterflied away - it was still in an embryonic state at the time of Muhammad's death and has not gathered enough followers and worldly success to survive. Yathrib (Medina) would be sacked by the Meccans, those of Muhammad's followers who are not killed would flee and disperse. However, it is unlikely that Muhammad's legacy would be forgotten entirely - memories would survive of a man preaching about the one God, gathering a small community of like-minded followers and eventually being killed by the Meccan pagans. If, a generation or so later, Axum or the Byzantine Empire move into the Hajiz (which they were seeking to do because the Hajiz is an important trade route) and the region becomes Christianized, it is possible that Muhammad would be declared a Christian martyr and a saint. In this scenario, then, Muhammad does not actually convert to Christianity, but is later declared a Christian through revision.
This is as far as my thoughts take me, for the time being. Post your feedback if you like, and let me know if you can think of other ways of how Muhammad could convert to Christianity.