Pre-Christian religious mysteries 
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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.
Christian Mysteries 
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.
The word mysterion (μυστήριον) is used 27 times in the New Testament. It denotes not so much the meaning of the modern English term mystery, but rather something that is mystical. In the biblical Greek, the term refers to "that which, being outside the unassisted natural apprehension, can be made known only by divine revelation".
For the Eastern Orthodox, Christian life is centered in the Mystery of the Incarnation of Christ, the union of God and man. However, the redemption of man is not considered to have taken place only in the past, but continues to this day through theosis. The Sacraments, or Sacred Mysteries are the most important means by which the faithful may obtain union with God, provided they are received with faith after appropriate preparation. Orthodox Christians believe that God is present everywhere and fills all things by his Divine grace, and that all of creation is, in some sense, a "sacrament". However, they believe that "He is more specifically and intensively present in [those] particular and reliable manners which He Himself has established," i.e., in the Sacred Mysteries.
Though Orthodox instructional materials may list seven Sacred Mysteries (Western names in parentheses) - Baptism, Chrismation (Confirmation), Confession (Penance, Reconciliation or Confession), Holy Communion (Eucharist or Holy Communion), Marriage (Holy Matrimony), Ordination (Holy Orders), and Unction (Anointing of the Sick. Archaic: Extreme Unction) - it must be understood that the term is not limited to these seven. As in the West, all faithful men are expected to receive six of the seven listed above, as well as either marriage or ordination, or both; women may not receive holy orders. The Sacred Mysteries can be defined as "those holy acts through which the Holy Spirit mysteriously and invisibly confers Grace (the saving power of God) upon man".
See also 
|Wikiquote has a collection of quotations related to: Mystery|
- Strong, James, The New Strong's Expanded Exhaustive Concordance of the Bible (Thomas Nelson Publishers, Nashville, TN, 2001, ISBN 0-7852-4539-1), p. 168.
- The Sacramental Life: An Orthodox Christian Perspective, (St. John of Kronstadt Press, Liberty, TN, 1986), p. 6.
- The Sacramental Life (1986), p. 7.
- Archpriest Seraphim Slobodskoy, The Law of God (Printshop of St. Job of Pochaev, Jordanville, NY, 1996, ISBN 0-88465-044-8), p. 471.