I know there are some huge butterflies, but let's focus on the early bit. Let's say that instead of being told to start his own religion, Mohammed instead is given the same type of visions that people in the Bible allegedly has and converts to Christianity.
Assume things go relatively OTL, he gets persecuted but eventually comes back and converts the whole city and starts an empire, which expands quickly and gains alot of converts.
How will the relationship be between the Arab Christians and the European Christians?