The American Church Dictionary and Cycopedia
A Truth or fact of Religion which has been revealed but not explained is called a mystery, because proposed to our faith faculty, such as the Incarnation, the Atonement, the Blessed Trinity, the Doctrine of the Eucharist. St. Paul speaks of the whole Revelation of Christ as the "Mystery of Godliness." Derived from the Greek word musterion, which in the Greek Church is the equivalent of our word "Sacrament."
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Miller, William James. Entry for 'Mystery'. The American Church Dictionary and Cyclopedia. http://www.studylight.org/dictionaries/acd/m/mystery.html. 1901.