A fact of crucial importance for the history which many of us share is that in late antiquity a large part of the population of the Roman Empire, perhaps even the majority, but in any case the dominant part of the population, converted to Christianity, and Christianity became the official religion of the Empire. Part of becoming a Christian, put in modern terms, is to become a monotheist, that is, roughly speaking, a person who believes in just one god, namely in God with a capital ‘G’, unless, of course, one already was a monotheist. But at least the vast majority of the inhabitants of the Roman Empire had been, again in modern terms, polytheists, that is, again roughly speaking, persons who believe in a plurality of gods. Given the importance of the fact that Christianity within the relatively short time of a few centuries came to be the dominant religion within the Empire, replacing, and for the most part eliminating, the various forms of paganism, one would like to know how this came about. Obviously there are answers to this from a religious perspective. But, since we are dealing with a remarkable historical phenomenon, we might want to understand how this came about in purely historical terms. From this perspective the change is bound to present itself as a highly complex matter.