Rome credited its religious constitution to the legendary king Numa, but its genius was absorption. Etruscan haruspices read entrails for the Senate; the Greek Olympians arrived early and merged with Latin gods, Zeus with Jupiter, Hera with Juno, Athena with Minerva; and in 204 BCE, on Sibylline advice, the city formally imported the Anatolian Magna Mater, black stone, eunuch priests and all. Conquest accelerated the traffic: Isis of Egypt gained temples in Rome over repeated Senate objections, and the soldiers' god Mithras, whose cult Manfred Clauss has reconstructed from its cave-temples, initiated men in grades from Raven to Father across the frontier provinces.
Empire added an imperial cult. From Augustus onward the genius of the living emperor and the divinity of deified predecessors received sacrifice throughout the provinces, a loyalty religion whose refusal marked Christians out for persecution. Judaism, ancient and licensed, was tolerated; the new movement that grew from it was intermittently suppressed for two and a half centuries, until Constantine's conversion reversed the state's direction in 312 CE.
The end came by law. Theodosius banned public sacrifice and closed the temples in the early 390s; the altar of Victory left the Senate house, the Vestal fire went out, and the old religion faded into the countryside customs its adversaries called paganus, the religion of the villages. Its calendar, its priestly titles and its Latin liturgical instinct passed, transformed, into the church that replaced it.