I think that Rodney Stark studies of new religious movements in the modern world give us some insights into the kinds of questions that we can ask about early Christianity.
[Christians] practiced moderation and chastity in marriage, for example, and Galen was puzzled at how they were able to do so. His comments are a kind of back-handed compliment, I suppose.
Galen's observations reflect one of the distinctive features of early Christianity. Roman-era religion was mainly about ritual actions vis-à-vis the gods, and there was little to do with ordering behavior otherwise.