Roman deity Jupiter, king of the gods.

During the Roman Republic, there was a strict system of priestly offices under the governance of the College of Pontiffs, with at its head the Pontifex maximus, which was the most important office. Flamens took care of the cults of various gods, while augurs were trusted with taking the auspices. The rex sacrorum, or "king of sacred things" took on the religious responsibilities of the deposed kings.

As contact with the Greeks increased, the influence of Greek religion was increasingly felt. The old Roman gods became associated and sometimes syncretized with Greek gods. Therefore the supreme sky-god Jupiter was perceived as being the same deity as Zeus. Mars was associated with Ares, Venus with Aphrodite, and the sea-god Neptune with Poseidon. Mercury, god of merchants and circulation, became associated with the Greek Hermes. Vesta, goddess of the hearth whose fire in her temple was not permitted to go out, was identified with Hestia. However, Jupiter had a distinctive Italic flavour that Zeus did not and Juno retained as much of her Etruscan forebear as she borrowed from the Greek Hera. Some gods were adopted from Greek religion. The first Roman temple to Apollo, a Greek god associated with music, healing, light and truth, was established in the 430s BC, replacing an older cult site of uncertain date.

The transference of the anthropomorphic qualities to Roman Gods, and the prevalence of Greek philosophy among well-educated Romans, brought about an increasing neglect of the old rites, and in the 1st century BC the religious importance of the old priestly offices declined rapidly, though their civic importance and political influence remained. Roman religion in the empire tended more and more to centre on the imperial house, and several emperors were deified after their deaths.

Under the Empire, religion in Rome evolved in many ways. Numerous foreign cults grew popular, such as the worship of the Egyptian Isis and the Persian Mithras. The importance of the imperial cult grew steadily, reaching its peak during the Crisis of the Third Century. Also, Christianity began to spread in the Empire, gaining momentum in the second century. Despite persecutions, it steadily gained converts. It became an officially supported religion in the Roman state under Constantine I. All cults except Christianity were prohibited in 391 by an edict of Emperor Theodosius I. However, even in the fourth and fifth century Roman paganism kept its vitality. Temples were still frequently visited, ancient beliefs and practices continued.

The divinity of the emperor and the cult surrounding him were a very important part of religion in the Roman Empire. In an effort to enhance political loyalty among the populace, they called subjects to participate in the cult and revere the emperors as gods. The emperors Augustus, Claudius, Vespasian, and Titus were deified, and after the reign of Marcus Cocceius Nerva, few emperors failed to receive this distinction.

The Roman religion in the empire tended more and more to center on the imperial house. Especially in the eastern half of the empire imperial cults grew very popular, and the cult complex became one of the focal points of life in the Roman cities. As such it was one of the major agents of romanization. The central elements of the cult complex were next to a temple; a theatre or amphitheatre for gladiator displays and other games and a public bath complex. Sometimes the imperial cult was added to the cults of an existing temple or celebrated in a special hall in the bath complex.

Evidence for the importance of the imperial cult include the "Achievements of the Divine Augustus" (Res Gestae Divi Augusti), written upon two large bronze pillars once located in Rome, Roman coins where the Emperor is portrayed with a halo or nimbus, and temple inscriptions such as "Divine Augustus Caesar, son of a god, imperator of land and sea..." (Roman Temple Inscription in Myra, Lycia).

As the Roman Empire expanded, and included people from a variety of cultures, more and more gods were incorporated into the Roman religion. The legions brought home cults originating from Egypt, Britain, Iberia, Germany, India and Persia. The cults of Cybele, Isis, Mithras, and Sol Invictus were particularly important. Some of those were initiatory religions of intense personal significance, similar to Christianity in those respects.

St. Peter and St. Paul introduced Christianity to the Romans, after Jesus died in c. 33 AD. Christian missionaries traveled across the empire, steadily winning converts and establishing Christian communities. After the Great Fire of Rome in July 64, Emperor Nero (56-68) accused the Christians as convenient scapegoats who were later persecuted and martyred. From that point on, Roman official policy towards Christianity tended towards persecution. The Roman authorities suspected Christians of disloyalty to the Emperor and of committing various crimes against humanity and nature. Persecution recurred especially at times of civic tensions and reach their worst under Diocletian (284 to 305).

Constantine I (324-337) ended the persecutions by establishing religious freedom through the Edict of Milan in 313. He later convened the historic First Council of Nicaea in 325, a year after ending the civil war of 324 and emerging as the victor in the war of succession. This First Council of Nicaea was formed to oppose Arius who had challenged the deity of Jesus Christ. The result was the branding of Arianism as a heresy. Catholic Christianity, as opposed to other religious groups, became the official state religion of the Roman empire on February 27, 380 through an edict issued by Emperor Theodosius I in Thessalonica and published in Constantinople. All cults, save Christianity, were prohibited in 391 by another edict of Theodosius I. Destruction of temples began immediately. When the Western Roman Empire ended with the abdication of Emperor Romulus Augustus in 476, Christianity survived it, with the Bishop of Rome as the dominant religious figure.

When Constantine became the sole Roman Emperor in 324, Christianity became the leading religion of the empire. After the death of Constantine in 337, two of his sons, Constantius II and Constans took over the leadership of the empire. Constans, ruler of the western provinces, was, like his father, a Christian. In 341, he decreed that all pre-Christian Graeco Roman worship and sacrifice should cease; warning those who still persisted in practicing ancient Graeco-Roman polytheism with the threat of the death penalty.

Lay Christians took advantage of new anti-Graeco-Roman polytheism laws by destroying and plundering the temples. Temples that survived were converted into Christian churches: the Pantheon is the most notable example, having once been a temple to all the gods and later becoming a church in honor of all the saints. Many of the buildings in the Roman Forum were similarly converted, preserving the structures if not their original intent.

Later on, the emperor Julian the Apostate attempted to reverse the process of Christianization and bring back the native forms of polytheism, but his death in Persia caused the empire to once again fall under the power of Christian control, this time permanently.