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.

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."

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).

Justin Martyr says that he was impressed in his study of the Old Testament, at the seriousness and contents of these writings, and became convinced that Jesus fulfilled these texts.

Justin's testimony about becoming a Christian is that he had been searching various philosophical traditions of the time, and then accidentally encountered a man who posed questions that pointed Justin [Martyr ] in a new direction.

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

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