The Qu'ran is largely based on the bible, mentioning a lot of biblical figures and events. Mohammed must have had rather detailed knowledge about the bible to be able to write it. But how exactly did Christianity find it's way to the tribal societies in Arabia? Was it brought there by missionaries? Who told Mohammed about what it said (as he couldn't read)?
Muhammad learned about Christianity from all around him.
First, he was a trader. Naturally, he dealt with a wide array of people.
Second, Christianity was already deeply ingrained in Arab society during Muhammad's time. There were large Christian communities living around Arabia, especially in Yemen and Hejaz. The prescience of Judaism and Christianity in Arab society during Muhammad's lifetime tells us that monotheism (what Muhammad preached) was not an alien concept to Arabs.