A mystery religion is a religion whose beliefs, practices, and true nature, are revealed only to those who have been initiated into its secrets.

Common components of these mystery cults (Latin 'mystai') included sacred symbols and rites with magical efficacy, purifications, asceticism, baptisms and sacraments. The highest promise of the mystai was a happy afterlife through salvation, which was conferred by the perennial and redemptive death of a "dying-and-rising" god and participation in a cult's mysteries. Initiation into the mysteries of a deity was divided into three stages through which an adherent had to ascend to obtain knowledge of the higher mysteries of a particular cult. The Hellenised world was filled with such mystery cults. in Athens alone it has been estimated that the largest number of mystai at one point in time reached six hundred. Indeed, those particularly moved by religil or religious zeal were at liberty to engage themselves in the mysteries of any number of deities at any given time, as the mystai were in a certain measure bound up with syncretism. Lucius Apuleius, writing in the second century C.E., sought to express his ultimate piety by revealing in a letter that he was an initiate (mystes) of "almost all of the Greek mysteries" available to him, revealing the open and tolerant nature between such cults. Many scholars have put this kind of religious fluidity down to the fact that the pagan mystai of antiquity were so highly syncretised that they taught much the same theologies, regardless of their respective deities.

Examples of mystery religions no longer practiced:

Examples of current mystery religions: