Archaic Roman religion, at least concerning the gods, was made up not of written narratives, but rather of complex interrelations between gods and humans. Unlike in Greek mythology, the gods were not personified, but were vaguely defined sacred spirits called numina. Romans also believed that every person, place or thing had its own genius, or divine soul. During the Roman Republic, Roman religion was organized under a strict system of priestly offices, which were held by men of senatorial rank. The College of Pontifices was uppermost body in this hierarchy, and its chief priest, the Pontifex Maximus, was the head of the state religion. Flamens took care of the cults of various gods, while augurs were trusted with taking the auspices. The sacred king took on the religious responsibilities of the deposed kings. In the Roman Empire, emperors were deified, and the formalized imperial cult became increasingly prominent.

As contact with the Greeks increased, the old Roman gods became increasingly associated with Greek gods. Thus, Jupiter was perceived to be the same deity as Zeus, Mars became associated with Ares, and Neptune with Poseidon. The Roman gods also assumed the attributes and mythologies of these Greek gods. Under the Empire, the Romans absorbed the mythologies of their conquered subjects, often leading to situations in which the temples and priests of traditional Italian deities existed side by side with those of foreign gods.

Beginning with Emperor Nero in the 1st century CE, Roman official policy towards Christianity was negative, and at some points, simply being a Christian could be punishable by death. Under Emperor Diocletian, the persecution of Christians reached its peak. However, it became an officially supported religion in the Roman state under Diocletian's successor, Constantine I, with the signing of the Edict of Milan in 313, and quickly became dominant. All religions except Christianity were prohibited in 391 CE by an edict of Emperor Theodosius I.