Well, it supposedly has more adherents than any other single religion, but it doesn't include a majority of the human race, so I don't know if "main" is the right word here. It is the plurality religion, though.

The process was very complex; there's no one-size-fits-all explanation. There are a number of general histories of Christianity, but in addition to a couple of those, I'd recommend reading "Conversion to Christianity from Late Antiquity to the Modern Age: Considering the Process in Europe, Asia, and the Americas," edited by Calvin Kendall and others. It contains chapters by leading scholars, each one detailing how and why people converted to Christianity in a particular place and time, over a period of centuries. No two of the stories are the same: it wasn't all "the king converted and ordered everybody else to" or "European colonizers brought missionaries with them" or any other single narrative. This book gives a great view of how diverse and unorganized the worldwide spread of Christianity has been.