Roman religion was not only polytheistic but treated foreign gods much as it treated foreign peoples: by incorporation. As far back as the takeover of Veii in the early fourth century BCE, Rome had regularly welcomed the gods of the conquered. There were from time to time controversies and anxieties about this; the priests of the Egyptian goddess Isis found themselves expelled from the city of Rome on more than one occasion. But the basic rule was that as the Roman Empire expanded, so did its pantheon of deities.
