The great period of the mystery religions began when the Romans imposed peace upon the Mediterranean world. The Dionysiac, or Bacchic, societies flourished in the whole empire—in Greece proper, on the Greek islands, in Asia Minor, along the Danube River, and especially in Italy and at Rome. Hundreds of inscriptions attest to Bacchic Mysteries. In some circles, Orphic and Dionysiac ideas were blended, as in the community that met in the underground basilica near the Porta Maggiore (Major Gate) at Rome. There was also a blend of ideas in the community for which the Orphic hymns were written. The members of this community (probably in Asia Minor) assembled at night in a clubhouse and held their services by the light of torches. Their rite consisted of a bloodless sacrifice and included the use of incense, prayer, and hymns. In addition to the mystery cults that were familiar from earlier times, the national religions of the peoples of the Greek Orient in their Hellenized versions began to spread. A faintly exotic flavour surrounded these religions and made them particularly attractive to the Greeks and Romans. The most popular of the Oriental mysteries was the cult of Isis. It was already in vogue at Rome in the time of the emperor Augustus, at the beginning of the Christian era. The Emperor, who wanted to restore the genuine Roman religious traditions, disliked the Oriental influences. But men of reputation, such as Messalla, a general and patron of writers, were strongly inclined toward the Isis Mysteries. Isis, the goddess of love, was the patroness of many of the elegant Roman courtesans. The religion of Isis became widespread in Italy during the 1st and 2nd centuries ad. To a certain extent, the expansion of Judaism and Christianity over the Roman world coincided with the expansion of the Egyptian cults.

Far less important was the influence of cults from Asia Minor. By 200 bc the Great Mother of the Gods (Magna Mater) and her consort Attis were introduced into the Roman pantheon and were considered as Roman gods. Their cult seems to have been encouraged especially under Emperor Claudius about ad 50. The Great Mother was characterized by her universal motherhood, especially over wild nature. The mysteries symbolized, through her relationship to Attis, the relations of Mother Earth to her children and were intended to impress upon the mystēs the subjective certainty of having been united in a special way with the goddess. There was a strong element of hope for an afterlife in this cult. The Persian god Mithra (Mithras), the god of light, was introduced much later, probably not before the 2nd century. The cult of Mithra was concerned with the origin of life from a sacred bull that was caught and then sacrificed by Mithra. According to Persian sources, the bull by its death gave birth to the sky, the planets, the earth, the animals, and the plants; thus Mithra became the creator of life. From Syria came the worship of several deities, of which Jupiter Heliopolitanus (the local god of Heliopolis; modern Baʿlabakk, Lebanon) and Jupiter Dolichenus (the local god of Doliche in Commagene; modern Dülük, Turkey) were the most important. Adonis (a god of vegetation) of Byblos (in modern Lebanon) had long been familiar to the Greeks and was often considered to be closely related to Osiris; the myths and rituals of the two gods were similar. Adonis’ female partner was Atargatis (Astarte), whom the Greeks identified with Aphrodite. At the time of Emperor Marcus Aurelius, in the latter half of the 2nd century ad, a pseudo-prophet named Alexander the Paphlagonian devised a great mystery spectacle centred around a holy snake called Glycon and had great success during his lifetime.

The height of Syrian influence was in the 3rd century ad when Sol, the Syrian sun god, was on the verge of becoming the chief god of the Roman Empire. He was introduced into Rome by the emperor Elagabalus (Heliogabalus) in about ad 220, and by about ad 240 Pythian Games (i.e., festivals of the sun god Apollo Helios) were instituted in many cities of the empire. The emperor Aurelian (270–275) elevated Sol to the highest rank among the gods. Sanctuaries of Sol and the gods of other planets (septizonium) were constructed. Even the emperor Constantine the Great, some 50 years later, wavered between Sol and Christ. For some time his religious policy was devised so as to allow the coexistence of both religions. Finally, Christianity was accepted as the official religion.

The different mystery religions were not exclusive of one another, but they appealed to different sociological groups. The middle class of the Greek and Roman cities preferred the Dionysiac societies, the festivals of which were a cult of beauty and merriment. Isis was worshipped by lower middle class people in the seaports and trading towns. The followers of the Great Mother in Italy were principally craftsmen. Mithra was the god of soldiers and of imperial officials and freedmen. There were no special societies for slaves; but they were usually admitted to the societies, and, during the time of the festival, all men were considered equal.