In the traditional religion of Rome, ritual was of great importance, for it established right relations with the gods. There were innumerable deities, and among the greatest were the Capitoline triad: Jupiter, a sky deity and king of the gods; Juno, his wife; and Minerva, goddess of wisdom. Mars, another major deity and the god of battle, was once a god of the fields, for the farmers of Rome were also its soldiers.

The family cult involved the propitiation of the gods of the household - Vesta (the hearth), the Lares (ancestral spirits) and Penates (the larder); it also formed the basis of the state cult, which was carried out on behalf of the people by the chief priests and magistrates. The priests were members of the aristocracy and were organized in colleges to conduct the rituals and arrange the festivals. These were agricultural to begin with, although new ones were added. The chief college of priests, that of the pontifices, took care of the records and accumulated lore; the augurs were responsible for divination; and the Vestal Virgins tended the hearth fires of the state.

The Romans also adopted the gods and cults of other people, particularly the Greeks, and sometimes identified local gods of their own. Abstract concepts, such as Victory or Fortune, were represented as gods and had also been adopted from the Greeks. One of the earliest foreign cults to come to Rome, in 204 BC, was that of the Phrygian fertility goddess Cybele. But the orgiastic rites so shocked the Senate that for a time Romans were forbidden to take part in them. The cult of Mithras was popular with soldiers, and the worship of the Egyptian goddess Isis and the god Serapis also became widespread.