Varrese Painter in the context of "Pelike"

Mystery religions, mystery cults, sacred mysteries or simply mysteries (Greek: μυστήρια), were religious schools of the Greco-Roman world for which participation was reserved to initiates (mystai). The main characteristic of these religious schools was the secrecy associated with the particulars of the initiation and the ritual practice, which may not be revealed to outsiders. The most famous mysteries of Greco-Roman antiquity were the Eleusinian Mysteries, which predated the Greek Dark Ages. The mystery schools flourished in Late Antiquity; Emperor Julian, of the mid-4th century, is believed by some scholars to have been associated with various mystery cults—most notably the mithraists. Due to the secret nature of the schools, and because the mystery religions of Late Antiquity were persecuted by the Christian Roman Empire from the 4th century, the details of these religious practices are derived from descriptions, imagery and cross-cultural studies.

Justin Martyr in the 2nd century explicitly noted and identified them as "demonic imitations" of the true faith; "the devils, in imitation of what was said by Moses, asserted that Proserpine was the daughter of Jupiter, and instigated the people to set up an image of her under the name of Kore" (First Apology). Through the 1st to 4th century, Christianity stood in direct competition for adherents with the mystery schools.