The indigenous Italic religion, which was the nucleus of the religion of ancient Rome, was essentially animistic. It depended on the belief that forces or spirits, called numina (sing., numen), existed in natural objects and controlled human destiny.
In the beginning of the historical period, when Italy was dotted with small agricultural communities, the family and the household were the basic religious units. Everything vital to the continuance of human life had its numen and appropriate rite. For the perpetuity of the family, the Italian farmer made offerings to the genius of the family. For the safety of the household he worshiped Vesta, the guardian spirit of the hearth fire; the lares and penates, guardians of the house; and Janus, guardian of the door. To protect the boundaries of his property he honored Terminus. To insure an abundant harvest he held various festivals throughout the year. To placate the spirits of the dead he made offerings to the lemures, to the manes, and to the deities of the underworld. In performing these religious ceremonies the head of the family acted as the priest and was assisted by his sons and daughters.
When these families coalesced into tribes and then a state, the family cult and ritual formed the basis of the state cult and ritual. Vesta had a community hearth, the penates a community storeroom, Janus a holy door in the Forum. Rome, which was theoretically one family, was ruled by its king, who as such was head of the family and chief priest. The king was assisted in his duties by his "sons and daughters," the colleges of priests and priestesses. They elaborated and recorded the rituals necessary for the propitiation of the gods and regulated the state ceremonies and the ceremonial calendar. The official clergy included the pontifex maximus, the rex sacrorum [king of the sacred rites], the pontifices, the flamens (see flamen), and the vestal virgins.
In the earliest period of Roman state religion, Jupiter, Mars, and Quirinus were the supreme triad. The Romans, however, tolerant of new gods and religions (provided that no harm was done to the state as such), adopted many foreign gods. Under the influence of the Etruscans and other Italic communities, new gods began to appear about the 7th cent. B.C. A wider and much more significant influence, however, was that of the Greek and Middle Eastern cults from about the 3d cent. B.C. Old Roman deities were equated with the Greek gods and accordingly endowed with their attributes and myths. Such important cults as the worship of Dionysus and Apollo were brought to Rome. Greek philosophy, particularly that of the Epicureans (see Epicurus) and the Stoics (see Stoicism), began to influence Roman religious thought.
In the last two centuries of the republic—when the old basis of Roman religion had lost much of its importance, and when the state had grown so massive and distant that its ceremonies failed to satisfy the populace—religious feeling rapidly degenerated. The people, needing a new and emotionally more satisfying religion, turned toward the religious mysteries and the Middle Eastern cults. The most prominent were those of the Great Mother (see Cybele), Isis and Osiris, Sol, and Mithra. Old Roman worship had been controlled, impersonal, and concerned with matters of the everyday world. The new cults, which centered around the individual, promised personal salvation and blessed afterlife. It was in this religious air that Christianity took root and eventually triumphed.
See W. R. Halliday, Lectures on the History of Roman Religion (1922); F. Altheim, A History of Roman Religion (1938); H. J. Rose, Ancient Roman Religion (1959).