'Mystery' in the Bible

even the mystery which hath been hid for ages and generations: but now hath it been manifested to his saints,

to whom God was pleased to make known what is the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory:

that their hearts may be comforted, they being knit together in love, and unto all riches of the full assurance of understanding, that they may know the mystery of God, even Christ,

withal praying for us also, that God may open unto us a door for the word, to speak the mystery of Christ, for which I am also in bonds;