In the Marquette Lecture volume, I focus on the question in the title. I emphasize the social and political costs of being a Christian in the earliest centuries, and contend that many attempts to answer the question are banal. I don't attempt a full answer myself, but urge that scholars should take the question more seriously.

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.

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."

Studies of the people named and described in earliest Christian texts show that, right from the earliest years, they included craftsmen, merchants, and owners of businesses. Of course, there were also slaves and poor among believers. By at least the second century, there were also believers from upper levels of Roman society.

"Religions" of the Roman world varied. Most of it was ritual practices. There were periodic occasions when cities honored their guardian deities with sacrifices and ceremonies, to ensure that they stayed happy and kept the city safe.

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.