The Roman Empire's religious diversity under the Principate is one of the more remarkable aspects of its functioning. The official state religion (sacrifices to the traditional Roman gods and to the genius of the emperor) coexisted with mystery cults, philosophical schools, Jewish communities, and eventually Christianity across the empire's vast territory.

The mystery religions — Mithraism, the Isis cult, the Cybele cult — are the religious forms that directly competed with early Christianity and whose eventual defeat has shaped how we think about late antique religion. They shared with Christianity an emphasis on personal salvation, initiation rituals, and communal identity. They differed in lacking Christianity's exclusive truth claim and institutional organization.

Mithraism is particularly interesting because its archaeological evidence — the mithraea (underground sanctuaries) found across the empire, especially along the Rhine and Danube frontiers — is abundant, but its literary documentation is sparse. We know what the cult looked like from archaeology; we know almost nothing about what its adherents believed, because no Mithraic theological text survives.

The cult of Isis, originating in Egypt, is better documented. The Metamorphoses (Golden Ass) of Apuleius gives us the only novelistic account of an ancient mystery cult initiation — the narrator's transformation back into human form through Isis' intervention. The reliability of this as a description of actual cult practice is debated, but it's the most vivid account we have.