Any religion with an arcanum, or body of secret wisdom. In a mystery religion, 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.

a belief system based around a mystery In medieval Britain , mystery meant a craft , trade or profession

a religion the true nature of whose beliefs and practices is known only to those who have been initiated into its secrets

a religion where the rules are not undisputable facts that were handed down by God/the Pope/the Grand poohbah, the rules and lessons of the religion are instead self-evident aspects of the world around us

a religion with an arcanum , or body of secret wisdom

A mystery religion is any religion with an arcanum, or secret wisdom. In a mystery religion, an inner core of beliefs, practices, and the religion's true nature, are revealed only to those who have been initiated into its secrets. (The ancient Greek term μυστήρια (mysteria) means "initiation", notably in the context of the Eleusinian Mysteries.)