A mystery religion is a religion with an arcanum, or body of secret wisdom. An inner core of beliefs, practices, and the religion's true nature, are revealed only to those who have been initiated into its secrets. These secrets are not to be profaned (pro + fanum "beyond the fane, or religious precinct"). By their very nature mysteries are ineffable, beyond what can be expressed in words, or esoteric (as in Gnosticism) (gnosis, "knowledge"), so mysteries do not produce a body of scripture that is claimed to be "revealed" by the prophet of a deity. Compare the public scriptural revelations that are characteristic of any "revealed religion" and the seasonally shared public cult practices of a "natural religion."

Common cult components of a mystery (Latin misterium) include sacred symbols and rites with personal spiritual and magical efficacy, purification rituals that may include abstinence from taboo foods or actions (compare asceticism), baptisms and other initiation rites and sacraments. Mysteries provide a threshold to a beatific vision. The highest promise for the mystai (Greek, "initiates") was the comprehension of the natural life-death-rebirth cycle as it was evoked through participation in a cult's mysteries, and in some mystery religions even a blissful afterlife through salvation, which was conferred by the perennial and redemptive death of a "dying-and-rising" god.

Ironically, part of what we know of the Eleusinian Mysteries comes from the scoffing descriptions recorded by late 4th and 5th century Christian monks, whose double purpose was to degrade and belittle the mysteries through burlesque, and at the same time to profane them by publishing them, much as fanes were desecrated, altars smashed and oracles silenced. The Eleusinian Mysteries ended in 396 CE when Christians directed Alaric to the little village of Eleusis near Athens, as related by Eunapios. *List of Buddhist topics

Initiation into the mysteries of a deity might be divided into several stages through which an adherent had to ascend to obtain knowledge of the higher mysteries of a particular cult. Mithraism had seven stages towards ultimate understanding. The Hellenistic world was filled with such mystery cults. In Athens alone it has been estimated that the largest number of mystai of the Eleusinian Mysteries at one point in time reached six hundred. Nevertheless, those who felt themselves particularly inspired were at liberty to engage themselves in the mysteries of more than one deity in their lifetime, for the syncretism of the pagan world was not exclusive in its devotions. There were many redeemers: Christians were unique in professing the One Redeemer, a characteristic that did not endear them to their neighbors. In The Golden Ass (2nd century CE) Lucius Apuleius's narrator sought to express his ultimate piety by revealing that he was an initiate (mystes) of "almost all of the Greek mysteries" available to him, revealing the open and tolerant nature among such cults. Only the extent of the narrator's enthusiasm was meant as comedy. 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.

In the language of the early Christians the mysteries were those religious teachings that were carefully guarded from the knowledge of the profane. An example is the Secret Gospel of Mark, which was preserved from profane view in Alexandria, and is now known only through chance references in a letter of Clement of Alexandria. The "sayings" Gospel of Thomas expresses mysteries that were confided by Jesus to Thomas alone, according to the manuscript, and the traditions of early Christian Gnosticism were based to esoteric information available only to disciples. Though these traditions were marginalized by mainstream Pauline Christianity and eventually declared "heretical", it has been suggested that Christianity had its origin in a mystery of initiates. According to this view, Christianity began as a Jewish adaptation of Greek mystery religion, and that Paul drew organized Christianity in another, more public, Hellenized direction, ultimately more acceptable to mainstream Roman culture.

Examples of mystery religions no longer practiced:

• The Eleusinian Mysteries
• Pythagoreanism
• The Cult of Isis
• Mithraism
• Gnostic Christianity

Examples of current mystery religions, in which gnosis is confided to initiates:

• The Druze religion
• The Church of Jesus Christ of Latter-day Saints (Mormons)
• Scientology
• Santería
• Candomble
• Gnosticism
• Occultism