Maybe Mohammed's life goes as per OTL, but the Muslim invaders don't do as well in the West as in OTL, and they are never in control of historically Christian Egypt or Anatolia, or of the Holy Land. But they do about as well in the East as in OTL. This makes it so that Christians and Muslims are never at odds with each other after the very beginning, making warmer relations easier to come by. Eventually, there is a major marriage between the royalties of a Christian and Muslim kingdom for political reasons. The family begins practicing both Christian and Muslim traditions and beliefs, and eventually the trend begins to catch on.
OR
Maybe the Crusades are a massive success, with Arabia being occupied due to the zeal of the Crusaders, and then eventually the whole of the Islamic World. The ongoing holy war keeps Christian Europe united, but eventually the sheer difficulty of keeping the occupation going proves too much. Trying to keep the empire while making the population easier to control, Mohammed is made a Saint in the Christian religion and other concessions are made, amking the population less unclined to revolt. This has the unintended consequence of European Christians adopting some Muslim practices in worship, and after hundreds and hundreds of years, Islam and Christianity become indistinguishable.
Don't know how likely either of those scenarios are, though.