Romans 16:25 (KJV)
Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
1 Corinthians 2:7-14 (KJV)
6 Yet among the mature we do impart wisdom, although it is not a wisdom of this age or of the rulers of this age, who are doomed to pass away.
7 But we speak the wisdom of God in a mystery, [even] the hidden [wisdom], which God ordained before the world unto our glory:
8 None of the rulers of this age understood this, for if they had, they would not have crucified the Lord of glory.
9 But, as it is written, "What no eye has seen, nor ear heard, nor the heart of man imagined, what God has prepared for those who love him"- 
10 these things God has revealed to us through the Spirit. For the Spirit searches everything, even the depths of God.
11 For who knows a person's thoughts except the spirit of that person, which is in him? So also no one comprehends the thoughts of God except the Spirit of God.
12 Now we have received not the spirit of the world, but the Spirit who is from God, that we might understand the things freely given us by God.
13 And we impart this in words not taught by human wisdom but taught by the Spirit, interpreting spiritual truths to those who are spiritual.
14 The natural person does not accept the things of the Spirit of God, for they are folly to him, and he is not able to understand them because they are spiritually discerned.
Colossians 1:26 (KJV)
[Even] the mystery which hath been hid from ages and from generations, but now is made manifest to his saints:
Colossians 2:2 (KJV)
That their hearts might be comforted, being knit together in love, and unto all riches of the full assurance of understanding, to the acknowledgement of the mystery of God, and of the Father, and of Christ;