The priesthoods of public religion were held by members of the elite classes. There was no principle analogous to separation of church and state in ancient Rome. During the Roman Republic (509–27 BC), the same men who were elected public officials might also serve as augurs and pontiffs. Priests married, raised families, and led politically active lives. Julius Caesar became pontifex maximus before he was elected consul.
The augurs read the will of the gods and supervised the marking of boundaries as a reflection of universal order, thus sanctioning Roman expansionism as a matter of divine destiny. The Roman triumph was at its core a religious procession in which the victorious general displayed his piety and his willingness to serve the public good by dedicating a portion of his spoils to the gods, especially Jupiter, who embodied just rule. As a result of the Punic Wars (264–146 BC), when Rome struggled to establish itself as a dominant power, many new temples were built by magistrates in fulfillment of a vow to a deity for assuring their military success.
As the Romans extended their dominance throughout the Mediterranean world, their policy in general was to absorb the deities and cults of other peoples rather than try to eradicate them, since they believed that preserving tradition promoted social stability. One way that Rome incorporated diverse peoples was by supporting their religious heritage, building temples to local deities that framed their theology within the hierarchy of Roman religion. Inscriptions throughout the Empire record the side-by-side worship of local and Roman deities, including dedications made by Romans to local gods.
By the height of the Empire, numerous international deities were cultivated at Rome and had been carried to even the most remote provinces, among them Cybele, Isis, Epona, and gods of solar monism such as Mithras and Sol Invictus, found as far north as Roman Britain. Foreign religions increasingly attracted devotees among Romans, who increasingly had ancestry from elsewhere in the Empire. Imported mystery religions, which offered initiates salvation in the afterlife, were a matter of personal choice for an individual, practiced in addition to carrying on one's family rites and participating in public religion. The mysteries, however, involved exclusive oaths and secrecy, conditions that conservative Romans viewed with suspicion as characteristic of "magic", conspiratorial (coniuratio), or subversive activity. Sporadic and sometimes brutal attempts were made to suppress religionists who seemed to threaten traditional morality and unity, as with the senate's efforts to restrict the Bacchanals in 186 BC. Because Romans had never been obligated to cultivate one god or one cult only, religious tolerance was not an issue in the sense that it is for competing monotheistic systems. The monotheistic rigor of Judaism posed difficulties for Roman policy that led at times to compromise and the granting of special exemptions, but sometimes to intractable conflict. For example, religious disputes helped cause the First Jewish–Roman War and the Bar Kokhba revolt.
In the wake of the Republic's collapse, state religion had adapted to support the new regime of the emperors. Augustus, the first Roman emperor, justified the novelty of one-man rule with a vast program of religious revivalism and reform. Public vows formerly made for the security of the republic now were directed at the well-being of the emperor. So-called "emperor worship" expanded on a grand scale the traditional Roman veneration of the ancestral dead and of the Genius, the divine tutelary of every individual. The Imperial cult became one of the major ways in which Rome advertised its presence in the provinces and cultivated shared cultural identity and loyalty throughout the Empire. Rejection of the state religion was tantamount to treason. This was the context for Rome's conflict with Christianity, which Romans variously regarded as a form of atheism and novel superstitio. Ultimately, Roman polytheism was brought to an end with the adoption of Christianity as the official religion of the empire.