The term sacred mysteries generally denotes the area of supernatural phenomena associated with a divinity or a religious ideology.

The mystery religions of antiquity were religious cults which required initiation before a participant was accepted. They included the Eleusinian Mysteries, Mithraism, the Cult of Isis, and the Cult of Sol Invictus. Mystery traditions were popular in ancient Greece and during the height of the Roman Empire.

The term is used in Eastern Christianity to refer to what the Western Church currently calls Sacraments and Sacramentals. In the Early Church they were kept hidden from the pagans — the so-called Disciplina arcani — lest they become objects of ridicule. As the Age of Persecution ended, the secrecy was gradually relaxed. But the term continued to be used. Originally the term "Mystery" was used in both the East and the West, as shown from the "Mystagogical Homilies" of St. Cyril of Jerusalem and the work, "On the Mysteries" by St. Ambrose of Milan.

The terms Sacrament and Sacramental are terms which the Western Church has carefully defined in Canon Law. Thus, for instance, the Council of Trent declared there to be exactly seven sacraments. The Eastern Churches, in contrast, have never defined the Mysteries in such precise terms. And, though the Western Church teaches that the consecrated bread and wine of the Eucharist are one Sacrament, the Divine Liturgy refers to the Eucharist as the Mysteries, in the plural. Orthodox Christians have always received Holy Communion in both species (both the Body and the Blood), and even reserve both in the tabernacle.