The following is an excerpt from The MacArthur New Testament Commentary on Colossians 1.

that is, the mystery which has been hidden from the past ages and generations; but has now 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. (1:26–27)

The message Paul proclaimed in his ministry was the mystery which has been hidden from the past ages and generations; but has now been manifested to His saints. There are some things God reveals to no one. Deuteronomy 29:29 says, “The secret things belong to the Lord our God.” God reveals other things only to certain people. “The secret of the Lord is for those who fear Him” (Ps. 25:14). Proverbs 3:32 says, “He is intimate with the upright.” Still other things were hidden in the Old Testament but have now been revealed in the New. The New Testament calls them mysteries (musterion). Paul’s use of this word is not to indicate a secret teaching, rite, or ceremony revealed only to some elite initiates (As in the mystery religions), but truth revealed to all believers in the New Testament. This truth, that has now been manifested to His saints, is that which has been hidden from the past ages and generations, namely the Old Testament era and people. Now refers to the time of the writing of the New Testament. Such newly revealed truth includes the mystery of the incarnate God (Col. 2:2–3, 9); of Israel’s unbelief (Rom. 11:25); of lawlessness (2 Thess. 2:7; cf. Rev. 17:5, 7); of the unity of Jew and Gentile in the church (Eph. 3:3–6); and of the rapture (1 Cor. 15:51). This mystery truth is available only for those who are saints—true believers (cf. 1 Cor. 2:7–16). The phrase to whom God willed to make known clearly indicates that the mysteries are not discovered by the genius of man, but are revealed by the will and act of God. It is God’s purpose that His people know this truth.

Of all the mysteries God has revealed in the New Testament, the most profound is Christ in you, the hope of glory. The Old Testament predicted the coming of the Messiah. But the idea that He would actually live in His redeemed church, made up mostly of Gentiles, was not revealed. The New Testament is clear that Christ, by the Holy Spirit, takes up permanent residence in all believers (cf. Rom. 8:9; 1 Cor. 6:19, 20; Eph. 2:22). The revelation of the riches of the glory of this mystery among the Gentiles awaited the New Testament (Eph. 3:3–6). Believers, both Jew and Gentile, now possess the surpassing riches of the indwelling Christ (John 14:23; Rom. 8:9–10; Gal. 2:20; Eph. 1:7, 17–18; 3:8–10, 16–19; Phil. 4:19). The church is described as “the temple of the living  God; just as God said, ‘I will dwell in them and walk among them; and I will be their God, and they shall be My people’ ” (2 Cor. 6:16). That Christ indwell-s all believers is the source for their hope of glory and is the subject or theme of the gospel ministry. What makes the gospel attractive is not just that it promises present joy and help, but that it promises eternal honor, blessing, and glory. When Christ comes to live in a believer, His presence is the anchor of the promise of heaven—the guarantee of future bliss eternally (cf. 2 Cor. 5:1–5; Eph. 1:13–14). In the reality that Christ is living in the Christian is the experience of new life and hope of eternal glory.