Romans 16:25-27 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, 26. But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: 27. To God only wise, be glory through Jesus Christ for ever. Amen. Written to the Romans from Corinthus, and sent by Phebe servant of the church at Cenchrea.
1 Corinthians 2:7-8 KJV 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. Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
1 Corinthians 15:50-53 KJV 50. Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; neither doth corruption inherit incorruption. 51. Behold, I shew you a mystery; We shall not all sleep, but we shall all be changed, 52. In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed. 53. For this corruptible must put on incorruption, and this mortal must put on immortality.
Ephesians 3:1-7 KJV For this cause I Paul, the prisoner of Jesus Christ for you Gentiles, 2. If ye have heard of the dispensation of the grace of God which is given me to you-ward: 3. How that by revelation he made known unto me the mystery; (as I wrote afore in few words, 4. Whereby, when ye read, ye may understand my knowledge in the mystery of Christ) 5. Which in other ages was not made known unto the sons of men, as it is now revealed unto his holy apostles and prophets by the Spirit; 6. That the Gentiles should be fellowheirs, and of the same body, and partakers of his promise in Christ by the gospel: 7. Whereof I was made a minister, according to the gift of the grace of God given unto me by the effectual working of his power.
There's much more. These are just a few samples.