As the empire expanded, and came to include people from a variety of cultures, the worship of an ever increasing number of deities was tolerated and accepted. The imperial government, and the Romans in general, tended to be very tolerant towards most religions and cults, so long as they did not cause trouble. This could easily be accepted by other faiths as Roman liturgy and ceremonies were frequently tailored to fit local culture and identity.
An individual could attend to both the Roman Gods representing his Roman identity and his own personal faith, which was considered part of his personal identity. There were periodic persecutions of various religions at various points in time, most notably that of Christians. As the historian Edward Gibbon noted, however, most of the recorded histories of Christian persecutions come to us through the Christian church, which had an incentive to exaggerate the degree to which the persecutions occurred. The non-Christian contemporary sources only mention the persecutions passingly and without assigning great importance to them.
In an effort to enhance loyalty, the inhabitants of the empire were called to participate in the Imperial cult to revere (usually deceased) emperors as demigods. Few emperors claimed to be Gods while living, with the few exceptions being emperors who were widely regarded at the time to be insane (such as Caligula). Doing so in the early empire would have risked revealing the shallowness of what the emperor Augustus called the "restored republic" and would have had a decidedly eastern quality to it. It was, for example, his attempt to make himself a god (in the mold of the kings of the Achaemenid Persian Empire, which he had conquered) that helped to turn Alexander the Great's troops against him. Since the tool was mostly one the emperor used to control his subjects, its usefulness was greatest in the chaotic later empire, when the emperors were often Christians and unwilling to participate in the practice.
Usually, an emperor was deified after his death by his successor in an attempt by that successor to enhance his own prestige. This practice can be misunderstood, however, since "deification" was to the ancient world what canonization is to the Christian world. Likewise, the term "God" had a different context in the ancient world. This could be seen during the years of the Roman Republic with religio-political practices such as the disbanding of a senate session if it was believed the Gods disapproved of the session or wished a particular vote. Deification was one of the many honors a dead emperor was entitled to, as the Romans (more than modern societies) placed great prestige on honors and national recognitions.
The importance of the Imperial cult slowly grew, reaching its peak during the Crisis of the Third Century. Especially in the eastern half of the empire imperial cults grew very popular. 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.
The seriousness of this belief is unclear. Some Romans ridiculed the notion that a Roman emperor was to be considered a living god, or would even make fun of the deification of an emperor after his death. Seneca the Younger parodied the notion of apotheosis in his only known satire The Pumpkinification of Claudius, in which the clumsy and ill-spoken Claudius is not transformed into a god, but into a pumpkin. In fact, bitter sarcasm was already effected at Claudius' funeral in 54.
Absorption of foreign cults
Since Roman religion did not have a core belief that excluded other religions several foreign gods and cults became popular.
The worship of Cybele was the earliest, introduced from around BC 200. Isis and Osiris were introduced from Egypt a century later. Bacchus and Sol Invictus were quite important and Mithras became very popular with the military. Several of these were Mystery cults. In the first century BC Julius Caesar granted Jews the freedom to worship in Rome as a reward for their help in Alexandria.
Druids were seen as essentially non-Roman: a prescript of Augustus forbade Roman citizens to practice "druidical" rites. Pliny reports that under Tiberius the druids were suppressed—along with diviners and physicians—by a decree of the Senate, and Claudius forbade their rites completely in AD 54.
While Judaism was largely accepted, it was on occasion subject to (mostly) local persecution.
Until the rebellion in Judea in AD 66, Jews were generally protected. To get around Roman laws banning secret societies and to allow their freedom of worship, Julius Caesar declared Synagogues were colleges. Tiberius forbade Judaism in Rome but they quickly returned to their former protected status. Claudius expelled Jews from the city however the passage of Suetonius is ambiguous: "Because the Jews at Rome caused continuous disturbances at the instigation of Chrestus he [Claudius] expelled them from the city" . Chrestus has been identified as another form of Christus; the disturbances may have been related to the arrival of the first Christians, and that the Roman authorities, failing to distinguish between the Jews and the early Christians, simply decided to expel them all.
Christianity, originally a Jewish religious sect, emerged in Roman Judea in the first century AD. The religion gradually spread out of Judea, initially establishing major bases in first Antioch, then Alexandria, and over time throughout the Empire. For the first two centuries, the imperial authorities largely viewed Christianity simply as a Jewish sect rather than a distinct religion. Suetonius mentions passingly that: "[during Nero's reign] Punishments were also inflicted on the Christians, a sect professing a new and mischievous religious belief" but he does not explain for what they were punished.
Tacitus reports that after the Great Fire of Rome in AD 64 some in the population held Nero responsible and that to diffuse blame, he targeted and blamed the Christians. The war against the Jews during Nero's reign, which so destabilized the empire that it led to the first civil war since the days of Julius Caesar and Mark Antony, as well as Nero's suicide, plausibly provided an additional rationale for persecutions against this 'Jewish' sect.
Persecution of Christians would be a recurring theme in the Empire for the next two centuries. Eusebius and Lactantius document the last great persecution of the Christians under Diocletian at the beginning of the 4th century at the urging of Galerius. This was the most vicious persecution of Christians in the Empire's history. After Diocletian, however, the fact that emperors were often Christians themselves lessened whatever persecutions may have still been occurring.
As the 4th century progressed, Christianity had become so widespread that it became officially tolerated, then promoted (Constantine I), and in 380 established as the Empire's official religion (Theodosius I). By the 5th century Christianity had become the Empire's predominant religion rapidly changing the Empire's identity even as the Western provinces collapsed. This would lead to the persecution of the traditional polytheistic religions that had previously characterized most of the Empire.