In common English usage, something not understood, an enigma; but in the Bible, something hitherto secret in the mind of God but now disclosed. In Daniel (as also in the Qumran scrolls) the secret is God's plan for a coming new age when the wicked will be punished and the righteous rewarded. The divine mysteries are sealed in heaven until made known at the end of this age. In the NT the concept of mystery has taken an additional meaning with the belief that the Messiah has come and the kingdom of God has been inaugurated.
The previously hidden divine plan for the world has now been made known to the Gentiles (Rom. 16: 25–6); it is the mystery of God (Col. 2: 2) or the mystery of Christ (Eph. 3: 4). The Gentiles are now fellow heirs with the Jews in the privileges of the gospel. But whereas this mystery is apparently revealed to all alike in the Church in Col. (1: 26), in the later Eph. (3: 5) the revelation is for apostles and prophets only. Some would take this difference to be an argument for regarding Ephesians as unlikely to be from the same hand (Paul's) as Colossians.
Contemporary with the early Church were the pagan mystery religions in which initiates were admitted by a rite resembling baptism which promised immortality. The Eleusinian mysteries were Greek in origin, as were rites of Dionysus, who was believed to have descended into Hades and risen from the grave. Similar mystery religions were founded on the legends of Orpheus, or of Mithras and Adonis, gods who died and returned to life, and were the focus of annual commemorations of their dying and rising, with corresponding hopes offered to the initiates. After the NT era some of the features of these mystery religions may have been incorporated into Christianity and in 150 CE Justin Martyr observed the similarities of Christianity with Mithraism, and regarded them as Satanic counterfeits. But later the word ‘mysteries’ was used in the Church to designate the sacraments.