What if the early Christians had made a successful effort to convert the Arabs before Mohammed came along? It looks like everyone in the centuries between 300 (when Christianity was legalized) and 600 wrote the Arabs off as a difficult, scattered, and unimportant people. The Christian emperors of the Eastern Roman Empire seem not to have sent missionaries in that direction, preferring to buy off the Arab tribes now and then, to keep their occasional raids from becoming a significant distraction from the empire's struggle with its traditional enemy, the Persians.
What if, in his spiritual misery, Mohammed had been surrounded not by a polyglot paganism, but by a vibrant Christianity that had been going for a century or two?
Yes, things might have happened just as they did; he might still have wandered off to a cave and begun having those visions that have had such fateful and bloody consequences for the world.
But perhaps not. Perhaps there would have been a kindly parish priest to talk to. Perhaps there would have been a monastery nearby, where he could immerse himself in the already formidable Christian intellectual and spiritual heritage, and explore his spiritual yearnings with good guidance. Perhaps, as happened to countless others, he might simply have been helped to Christ by a Christian neighbor.
How many times did Christians think about evangelizing the Arabs during those centuries? How many times did they decide that those motley desert tribes just weren't worth it, that the Great Commandment somehow didn't apply to them?
The price of not spreading the Gospel can have repercussions stretching millennia into the future.