Colossians 1:26 [Even] the mystery which has been hid from ages and from generations, but now is made manifest to his saints: 1:27 To whom God would make known what [is] the riches of the glory of this mystery among the Gentiles; which is Christ in you, the hope of glory: 1:28 Whom we preach, warning every man, and teaching every man in all wisdom; that we may present every man perfect in Christ Jesus:
The law defines sin and right doing, but it cannot change the person's heart and character. It may outwardly restrain the evil passions in the carnal heart, but it doesn't change them.
This is where the mystery of God comes. The mystery of how a sinner, steeped in sinful thoughts, motives, and actions, is transformed into a new creature by the grace and power of God.
A mystery a person so that the sinful things that once appealed to him/her have lost their appeal, while the things of God which once they despised, now become their delight and enjoyment.
Paul tries to explain this mystery in his writings.
In 2 Cor. 3:1 he asks people if they expect him to carry letters of recommendation to convince people he is an authentic preacher of the gospel.
Then he writes:
2 You are our epistle written in our hearts, known and read by all men; 3 clearly you are an epistle of Christ, ministered by us, written not with ink but by the Spirit of the living God, not on tablets of stone but on tablets of flesh, that is, of the heart.
THE REAL RECOMMENDATION: Paul says, OUR letter or recommendation is YOU! And yet, he says, it is also written in OUR hearts. That is-- this letter of recommendation must be written on both sets of hearts. The true teacher of God has this letter written upon his heart, and the evidence that his teaching is true is the resulting LIFE of his listeners, which shows the letter written upon the recipients heart.
But Paul takes this further, for he speaks of the TABLETS OF STONE, and later of Moses. He speaks of "ministering"...
Paul and his helpers were the means God used for the Spirit to write God's gospel and God's law upon the hearts of the recipients in such a way that they become living testaments to the power and glory of God. People can read the writing of the Spirit of the living God in the fruit of this ministry revealed in the lives of the believers.
4 And we have such trust through Christ toward God. 5 Not that we are sufficient of ourselves to think of anything as being from ourselves, but our sufficiency is from God,
This sufficiency comes only from God. Paul says he has no sufficiency whatever and can claim nothing as his own product. All sufficiency is from God. Only a message sent from God could work such changes in the lives of people.