The best guess seems to be that, immediately before the emperors Constantine and Licinius issued the Edict of Milan in 313 AD, about 5 percent to 10 percent of the population of the Roman Empire was Christian. Although these percentages may seen low, they translate to millions of converts. Assuming a total population of about 55 million in the Roman Empire at the time, the number of Christians would have been somewhere between 2.75 million and 5.5 million.
And yet, as Ramsay MacMullen notes in Christianizing the Roman Empire A.D. 100-400, a profound mystery remains as to how Christianity had acquired so many converts. “After New Testament times and before Constantine,” there is almost no evidence of “open advertising” of Christianity, and much evidence that Christians were urged to lay low and to associate only with other Christians, both to avoid being identified in the event of persecution and in order to avoid the impure practices of the pagans.
How exactly, then, Prof. MacMullen wonders, did Christianity generate those millions of followers in the years before toleration? He suggests an answer by trying to “imagine in some detail a scene that conflicts with no point of the little that is known about conversion in the second and third centuries.”
I would choose the room of some sick person: there, a servant talking to a mistress, or one spouse to another, saying, perhaps: “Unquestionably they can help, if you believe. And I know, I have seen, I have heard, they have related to me, they have books, they have a special person, a sort of officer. It is true. Besides and anyway, if you don't believe, then you are doomed when a certain time comes, so say the prophecies; whereas, if you do, then they can help even in great sickness. I know people who have seen or who have spoken with others who have seen. And healing is even the least that they tell. Theirs is truly a God all-powerful. He has worked a hundred wonders.” So a priest is sent for, or an exorcist; illness is healed; the household after that counts as Christian; it is baptized; and through instruction it comes to accept the first consequences: that all other cults are false and wicked, all seeming gods, the same.