The reason for the popularity is a matter of scholarly speculation. Perhaps the population displacements, the exposure to foreign cults, and the breakdown of the city-state (polis) made people interested in change.
The gods of the Romans and the Greeks might have seemed out of touch with the new realities of empire and the need for community. The literature shows more attention to inward concepts like self, intimacy, personal relationships, and privacy, all terms that are not associated normally with Greek and Roman public religion.
Public religion bound all the citizens together by sacrifices that were openly conducted and enjoyed—that is to say, at altars outside the specific temple. Usually the sacrifice involved a feast day observed by everyone, processions that publicized the event, and finally a banquet where the sacrifice was consumed. All the citizens were bound together by such public demonstrations, and the bonding of everyone was more important than particular emotional expressions.
Exposure to the Middle East may have presented people with an alternative to the Greeks and Romans. There is evidence in documents and inscriptions that "hidden" teachings were passed on, perhaps from even more isolated or foreign groups (such as Persians, Egyptians, and Asians) in contact with the mediate cultures of the Middle East.
The Dead Sea Scrolls, for example, use the notion to promote the priority of their teaching. Certainly the New Testament (such as Col. 1:26), late jewish bible books (such as Daniel), and rabbinic Judaism (Moses’s "oral traditions") also speak of knowledge not known to mainstream religion. This idea also finds expression in the Gnostic sects of later centuries.
Often a small group of people would meet privately, and secret rituals would be conducted indoors away from the public eye. The Greek word mysterios means a secret that is revealed to insiders. Outsiders wrote about the secrets, many of whom were Christian and often hostile to or competing with the mystery cult.
Members were sworn to secrecy, and punishment was meted out to anyone who disclosed the mystery. The small group of the mystery cult emphasized their exclusive fraternity.
In order to belong, a process of initiation was set up. The initiation often was available only to certain qualified individuals, instead of to every interested person. The process of initiation might take days and hardships like fasting or vigils.
The idea was that the initiated member would experience solidarity with everyone else who endured the same experience. Through the initiation the members would feel a sense of identity and belonging in an otherwise foreign world.
Usually at the center of the mystery cult was a hero, who was the focus of the rituals. The activities of the cult served to reenact the life of the hero so that the members could participate and derive the strengths and virtues of the hero.
Often initiation began the participation, but there might be some ritual that ended or fulfilled the member’s process of initiation. Such things might involve sacred meals, dramas, or liturgies.
Many mystery cults of Roman times promised their members not only intimate community but union with the divine, liberation, and reassurances about the afterlife. Orpheus, Demeter, Dionysus, Achilles, Adonis, and others were the kind of heroes celebrated by cults.
They all shared in suffering, misery, or ill fate. In addition, they all were human (though mythical) and shared in human nature’s limitations, including loneliness and death. Thus, it was easier for the initiate to find solidarity with their hero than with the public religion’s gods and goddesses.
By the reenactment of the hero’s life, the participant might be able to purge his or her own anxieties and fears about life. As the mystery cults developed in the Roman world, the idea of "rebirth" replaced the idea of purgation of fear.
Some mysteries were considered deviant to public welfare and so were persecuted—and here Christianity might serve as an example. Public officials acknowledged other mysteries as serving a constructive and cohesive function for society.
The Eleusinian Mysteries conducted city-wide processions and inducted the likes of Marcus Aurelius, Alcibiades, and Julian the Apostate, and existed for more than 1,000 years before Theodosius I destroyed its sanctuary (400 c.e.) and established Christianity as the state religion. In general, the mystery cults did not openly contradict the public religion.
Membership in the mysteries was limited, though some permitted almost anyone regardless of rank and sex to join (Eleusis). Some mysteries served soldiers (Mithras), some women (Villa of Mysteries in Pompey), some family members or slaves (here many scholars would place Christianity).
Since secrecy and privacy surrounded the mystery religions, hard evidence for their members and rituals is lacking. The familiarity of the ancient world with the mystery cults may explain why Christianity came to be so readily accepted in the communities and societies outside of Diaspora Judaism.