"24 Now I rejoice in my sufferings for your sake, and in my flesh I am filling up what is lacking in Christ's afflictions for the sake of his body, that is, the church, 25 of which I became a minister according to the stewardship from God that was given to me for you, to make the word of God fully known, 26 the mystery hidden for ages and generations but now revealed to his saints. 27 To them God chose to make known how great among the Gentiles are the riches of the glory of this mystery, which is Christ in you, the hope of glory. 28 Him we proclaim, warning everyone and teaching everyone with all wisdom, that we may present everyone mature in Christ. 29 For this I toil, struggling with all his energy that he powerfully works within me.
2 For I want you to know how great a struggle I have for you and for those at Laodicea and for all who have not seen me face to face, 2 that their hearts may be encouraged, being knit together in love, to reach all the riches of full assurance of understanding and the knowledge of God's mystery, which is Christ, 3 in whom are hidden all the treasures of wisdom and knowledge. 4 I say this in order that no one may delude you with plausible arguments. 5 For though I am absent in body, yet I am with you in spirit, rejoicing to see your good order and the firmness of your faith in Christ."
Colossians 1:24 - 2:5
The second feature of this 'mystery' is that God has graciously chosen to make it known (v 27) 'among the Gentiles', for it related to them/us in a special way. This mystery of Christ, Paul says, reveals how non-Jews (Gentiles), along with Jews (the chosen people of God), are incorporated, on an equal footing, into His redemptive purposes (Ephesians 3:6). This is surely the glorious riches of God lavished upon us Gentiles. That we, the uncircumcised, we who were far off, separated from Christ, excluded from citizenship in Israel, strangers and foreigners to the covenant of promise, without hope and without God in the world; that, in Christ, we should be brought near through His blood and be made fellow citizens with the people of God and members of God's household. The wonder of the mystery is that, in Jesus Christ, salvation has come to us. Salvation has come to the wild and uncultured olive branches as, contrary to nature, we are grafted into the true olive tree. Thirdly, at the end of v 27, Paul identifies the content of this mystery, '...which is Christ in you, the hope of glory'. This is the mystery which is Paul's consuming passion. This is what Paul preached as the unsearchable riches of Christ, the Word of God in all its fullness, 'Christ in you the hope of glory'. The content, focus and centre of this mystery is Christ. This supreme and majestic, cosmic Christ whom Paul has just been exalting in the earlier section of his letter (1:15-23); this Christ who created the universe and who even now upholds it by the word of his power; He who by his death has accomplished cosmic, universal reconciliation; this Christ, mystery of mysteries, indwells the believer. He dwells within us. By His Spirit, the eternal King of heaven, who fills the vast universe, has taken up residence within our hearts: We have His resurrection life and power coursing through our veins. What a mystery! What a treasure!