The Roman Empire expanded to include different peoples and cultures; in principle, Rome followed the same inclusionist policies that had recognised Latin, Etruscan and other Italian peoples, cults and deities as Roman. Those who acknowledged Rome's hegemony retained their own cult and religious calendars, independent of Roman religious law. Newly municipal Sabratha built a Capitolium near its existing temple to Liber and Serapis. Autonomy and concord were official policy, but new foundations by Roman citizens or their Romanised allies were likely to follow Roman cultic models. Romanisation offered distinct political and practical advantages, especially to local elites. All the known effigies from the 2nd century AD forum at Cuicul are of emperors or Concordia. By the middle of the 1st century AD, Gaulish Vertault seems to have abandoned its native cultic sacrifice of horses and dogs in favour of a newly established, Romanised cult nearby: by the end of that century, Sabratha's so-called tophet was no longer in use. Colonial and later Imperial provincial dedications to Rome's Capitoline Triad were a logical choice, not a centralised legal requirement. Major cult centres to "non-Roman" deities continued to prosper: notable examples include the magnificent Alexandrian Serapium, the temple of Aesculapius at Pergamum and Apollo's sacred wood at Antioch.
According to an article by Claudia Moser (Metropolitan Museum of Art), Roman religion, both by native instinct and deliberate policy, was widely inclusive, comprised of different gods, rituals, liturgies, traditions, and cults. Romans, considered by Cicero as the religiosissima gens (the most religious peoples), not only worshipped their own traditional Latin gods and associated divinities imported from the culturally respectable and authoritative world of their Greek neighbors, but often acknowledged the gods of peoples they otherwise considered to be quite alien (such as, for example, the Semitic Aphrodite of Mount Eryx). They even annexed the gods of despised enemies, such as Carthage’s Tanit-Caelestis, in a process of evocation that assigned foreign gods Latin names. Between the late third century B.C. and the third century A.D., some eastern cults, such as those of Cybele (also known as Magna Mater), Isis, and Mithras, permeated the Roman world. These exotic cults differed from Judaism, another eastern religion, whose rites were “sanctioned by their antiquity” (Tacitus, Histories V.5) and which flourished throughout the Mediterranean world from the time of the Babylonian Captivity through the Roman diaspora and after. The cults of Cybele, Isis, and Mithras captivated Roman citizens with intriguing rituals and the promise of spiritual renewal in this world and salvation in the next.
Romans were particularly receptive to foreign cults at times of social upheaval, when old beliefs no longer provided answers to new uncertainties and fears. In 204 B.C., during the Second Punic War, the Romans consulted the Sibylline Oracles, which declared that the foreign invader would be driven from Italy only if the Idaean Mother (Cybele) from Anatolia were brought to Rome. The Roman political elite, in a carefully orchestrated effort to unify the citizenry, arranged for Cybele to come inside the pomerium (a religious boundary-wall surrounding a city), built her a temple on the Palatine Hill, and initiated games in honor of the Great Mother, an official political and social recognition that restored the pax deorum.
After Cybele and the foreign ways of her exotic priesthood were introduced to Rome, she became a popular goddess in Roman towns and villages in Italy. But the enthusiasm that accompanied the establishment of her cult was soon followed by suspicion and legal prohibitions. The eunuch priests (galli) that attended Cybele’s cult were confined in the sanctuary; Roman men were forbidden to castrate themselves in imitation of the galli, and only once a year were these eunuchs, dressed in exotic, colorful garb, allowed to dance through the streets of Rome in jubilant celebration (97.22.24). Nevertheless, the popularity of the goddess persisted, especially in the Imperial period, when the ruling family, eager to emphasize its Trojan ancestry, associated itself with and publicly worshipped Cybele, a goddess whose epithet, Mater Idaea, designated her as Trojan and whose cult was deeply connected with Troy and its origins.
Worship of the Egyptian mother goddess Isis was a popular alternative to the cult of Cybele (1991.76). By the middle of the first century A.D., with the political integration of the many lands bordering the eastern Mediterranean, the cult of Isis was transformed from a secret rite popular among the lower classes of Rome but not permitted within the sacred confines of the city, to a highly structured public cult closely associated with the emperors. During the reign of Vespasian, Isis was officially welcomed into the Roman pantheon, and a public temple within the sacred walls of the city was erected for her.
Although the cult of Isis, with its distinctive maternal and female characteristics, principally attracted women, the annual spring and autumn festivals held in her honor drew both sexes, of all classes, people celebrating different occasions and customs—springtime renewal, grief and joy. Plutarch describes the pervasive presence of the goddess and her exotic clothing: “the garments of Isis are dyed in rainbow colors, because her power extends over multiform matter that is subjected to all kinds of vicissitudes” (Isis and Osiris 77).
Unlike the public rituals and processions dedicated to Cybele and Isis in Imperial Rome, the worship of Mithras was secret and mysterious. At the end of the first century A.D., the Iranian god Mithras, creator and protector of animal and plant life, began to appear in Italy, becoming especially popular with Roman legionaries, imperial slaves, and ex-slaves. Not limited to the class of soldiers, however, Mithraists could also be found in the circles of the imperial households. In the absence of Mithraic literature, evidence of the cult, its rituals, and customs comes from archaeological finds and depictions of the god.
The religion of Mithras was practiced in small groups, with ten to twelve participants. Initiates into this secret cult immediately entered a priestly hierarchy, an order of seven grades, each with specific planets, costumes, rituals, and disciplines aimed at self-advancement. Members of the cult met in a mithraeum, an underground vaulted grotto with complex astronomical and planetary symbolism. The small space of the cavern, the cult practices, and the ritual meal were modeled on the original space and deeds of Mithras—the sacrifice of a bull and the eating of its flesh (1997.145.3).
The overall scarcity of evidence for smaller or local cults does not always imply their neglect; votive inscriptions are inconsistently scattered throughout Rome's geography and history. Inscribed dedications were an expensive public declaration, one to be expected within the Graeco-Roman cultural ambit but by no means universal. Innumerable smaller, personal or more secretive cults would have persisted and left no trace.
Military settlement within the empire and at its borders broadened the context of Romanitas. Rome's citizen-soldiers set up altars to multiple deities, including their traditional gods, the Imperial genius and local deities – sometimes with the usefully open-ended dedication to the diis deabusque omnibus (all the gods and goddesses). They also brought Roman "domestic" deities and cult practices with them. By the same token, the later granting of citizenship to provincials and their conscription into the legions brought their new cults into the Roman military.
Traders, legions and other travelers brought home cults originating from Egypt, Greece, Iberia, 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.
The Roman pantheon presented a wide range of cults and gods with different functions, but foreign cults promised something different, something the traditional Roman cults could not—change, in everyday life and even, at times, in the afterlife. The selectivity and initiatory rituals of these new cults fostered a strong sense of community, focusing on the religious affiliation rather than on the public status or race of an individual within the state. As the Roman author Ovid reported, “who would dare to drive from his doorstep one whose hand shakes the sonorous sistrum [of Isis]… when, before the Mother of the gods, the flute-player sounds his curving horn, who would refuse him alms of a copper coin” (Letters from the Black Sea I:1:37ff.) (45.16.2). By the end of the fourth century A.D., the official cults of Cybele, Isis, and Mithras, emblems of Roman paganism, were either completely suppressed or drastically altered and Christianity (an eastern cult as well, once called a “destructive superstition” by Tacitus) became the dominant religion of the Roman world.