Rome had always been a hotbed of religious diversity. While the official Roman gods were venerated across the Empire, Roman elites had no objections to the worship of other deities, and indeed many Romans (elites and commoners alike) eagerly embraced foreign faiths. Originating in the Hellenistic kingdoms, many Romans were attracted to mystery religions, cults that promised spiritual salvation to their members. These mystery religions shared a belief that the universe was full of magical charms that could lead to spiritual salvation or eternal life itself. In many ways, they were more like cults of magic than traditional religious faiths. A worshiper could join multiple mystery religions, intoning chants and prayers and participating in rituals in hopes of securing good fortune and wealth in life and the possibility of spiritual immortality after death.
Even Rome’s perennial adversary Persia supplied sources of spiritual inspiration to Rome. A Zoroastrian demigod, Mithras, became immensely popular among Romans. Mithrans believed that Mithras had been a soldier, slain by his enemies, who then rose to enjoy eternal life. Roman soldiers campaigning in Persia brought Mithranism back to Rome - Mithras’s identity as a former soldier made his worship all the more appealing to members of the Roman military. The worship of Mithras was so popular that, some historians have noted, it is easy to imagine the Roman Empire becoming Mithran instead of Christian if Constantine had not converted to the latter faith.
In some cases, non-Roman gods even came to supplant Roman ones; one of the Severan emperors embraced the worship of the Syrian sun god Sol Invictus (meaning "the unconquered sun") and had a temple built in Rome to honor the god alongside the traditional Roman deities. The notion of being as powerful and unstoppable as the sun appealed to future emperors, so subsequent emperors tended to venerate Sol Invictus along with the Roman Jupiter until the triumph of Christianity.