OK, DisillusionedJW - Here's some starters to think about.
1. I agree with Peter Heather's conclusions, Constantine's adoption of Xtianity as the Roman state religion. That spread Xtianity into much of Europe, North Africa and West Asia, though in the last two areas it did not survive the Islamic onslaught.
2. The rise of Xtian Western Europe as a colonial power, (as Bungi Bill's post suggests) Spain in particular was extremely successful in enforcing the adoption of Xtianity as the state religion in Central and South America and the Philippines.
There were failures of course, notably Japan where the RC church had established itself in Nagasaki and was starting to spread among the elite, and the government took the extreme step of trying to exterminate all Xtians. The French had more success in the parts of S.E Asia that it took control of at the beginning of the nineteenth C. But, the British attempt to Xtianise China also failed.