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Col. 1:26,27 "that is, the mystery which has been hidden from the past ages and generations; but now has been manifested to His saints, to whom God willed 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".

The mystery is "Christ in you".  Past ages had no idea that this was the plan, that Christ's own spirit would dwell in His people, doing away with the law.  Other places Paul describes the mystery a little bit differently, but all in all what it comes down to is the gospel.  The gospel itself is the mystery that God hid throughout the ages and then gave to His church the job of proclaiming.  Satan had no idea what it was.  Paul affirms that in several places, but my favorite is 1 Cor. 2:8 "the wisdom which none of the rulers of this age has understood; for if they had understood it, they would not have crucified the Lord of Glory".