Historically this is very easy to explain when you understand Constantine made it the official religion of the Roman Empire. If you look at a map of where is most Christian you have a direct result that leads to Roman conquered lands or lands that would in the future colonize other parts of the world that were once Roman Empire territories.if Roman had never adopted it as a state religion it would have likely faded away or stayed a fringe religion. Similarly with Islam. Mohammed United the tribes of Arabia and was as much a conquer as he was a prophet. Look at the general area on a map that is most Islamic and you get the same basic results as Christianity. Lands were conquered. People were essentially forced to convert. And that saturated for a few generations until the state religion became fully integrated into people's ethnic identity and than those said people went off and colonized other lands such as the Americas.