In Christianizing the Roman Empire (thanks for the book recommendation, Joel!), MacMullen tries to stay close to the written record in explaining why people converted to Christianity when it was illegal. He argues that most of the evidence suggests that non-Christians converted because miraculous cures and exorcisms showed them the power of God. Early Christian sources record that “exorcist” was a special office in the church. He tries to synthesize the evidence to conjecture about how this might have been lived out:
Testing to see if I can 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, they 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 the 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.” (40-41)
UPDATE (2/22/10): I added a world that I unwittingly omitted from MacMullen’s quote.