Key to the Mystery

What Exactly Is the Pauline “Revelation of the Mystery”?

“Now to him that is of power to stablish you according to my [Paul’s) 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: (Romans 16:25-26 (KJV)