I'd amend that to "Religions don't always spread peacefully" because the vast majority of Christian conversions were peaceful. For the first 300 years it was join AND die not join or die. Contrary to what most believe, after Constantine legalized it, there weren't any forced conversions. Most of the empire including Constantine himself did not convert. Even after the Emperors were Christian they still coexisted peacefully with pagans and the number in significant government positions roughly reflected the population demographics. That isn't to say that there haven't been horrors, but to say Christianity has spread mainly through violence is ahistorical.