[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 recognized, with some considerable puzzlement it seems, that Christians exhibited the virtues that he associated with the discipline of philosophy.

In its Roman-era setting, for example, Christianity was so different that critics of the time referred to it as a "superstition" (meaning a bogus or dangerous religion).

Some modern scholars contend that in terms of what "religion" was in that [Roman-era setting], you can't even call Christianity a religion.

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.

Early Christianity, like Roman-era philosophical traditions, laid emphasis on everyday behavior, about how to live your life.

I offer early Christianity as a case-study to show that the phenomena that we group under "religion(s)" comprise a somewhat artificial category, and that "religions" are not "all the same."