Mystery religions, mystery cults, sacred mysteries or simply mysteries, were religious schools of the Greco-Roman world for which participation was reserved to initiates (mystai). The main characterization of this religion is 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 school, 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. Much information on the Mysteries comes from Marcus Terentius Varro.

The Greco-Roman civilization, as understood by modern scholars and writers, includes the geographical regions and countries that culturally—and so historically—were directly and intimately influenced by the language, culture, government and religion of the Greeks and Romans. A better-known term is classical antiquity. In exact terms the area refers to the "Mediterranean world", the extensive tracts of land centered on the Mediterranean and Black Sea basins, the "swimming pool and spa" of the Greeks and the Romans, in which those peoples' cultural perceptions, ideas, and sensitivities became dominant in classical antiquity.