The Roman state's extraordinary and unexpected transformation from one that had hegemony over the greater part of Italy into a world state in the second and first centuries BCE had implications for Roman religion that are not easy to grasp. After all, Christianity, a religion wholly "foreign" in its origins, arose from this period of Roman ascendancy. To begin, then, to understand the religious system of imperial Rome, it is best to confine the discussion to some elementary and obviously related facts.

First, the old Roman practice of inviting the chief gods of their enemies to become gods of Rome (evocatio) played little part in the new stage of imperialism. Evocatio played some role in Rome's conquests in the middle Republic, but the practice had been transformed. The last temple to be built in Rome to house a deity "evoked" from an enemy of...