*supposed to be a reply to footbag, but a bunch of posts got in the way*
Christianity spread through Europe primarily through individual missionaries wandering through the land and setting up peaceful churches amongst Pagan communities. Pagan religions were usually very tolerant to different beliefs, especially compared to Christianity which was extremely intolerant to pretty much everything until recent times, and communities didn't start to fight back until a significant portion of them had been converted. At this point it was generally too late to stop the inevitable. After that you only need to move to the Bible Belt and put a sign saying "I DON'T BELIEVE IN GOD" to see how peaceful it was. You probably won't be physically assaulted... too often, but social ostracization is inevitable.
The Inquisition was a fairly nasty business, but along with witch hunts is totally blown out of proportions by people who don't know their history. In any case, neither had anything to do with how Christianity spread, which appears to be your main point.
The Crusades were more about greed that was justified by religion, but the Chruch openly encouraging people to kill the "infidels" sure didn't help one bit. Even if the major blame is placed on religion, it was more about reclaiming "their" holy land, not conversion which I still assume is what you're talking about here.
Hitler's religious beliefs are anybody's guess. Apparently he believed in some greater power, but certainly didn't subscribe to any mainstream Christian branch. Then again if he thought of himself as a Christian, then that makes him a Christian in my books. There have been so many different Christian sects over the millenia, that the only common feature amongst them is the veneration of a figure commonly known as Jesus Christ, who may or may not have been the son of god, may or may not have existed, and may or may not died for our sins.
Oh and in all fairness the persecution the early Christians suffered before becoming the dominant religion in the Roman Empire were nothing compared to what they did to Pagans, let alone "heretical" sects of their own faith.
