"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." Colossians 1:27
1:27 Gentiles … Christ in you. The OT predicted the coming of the Messiah and that the Gentiles would partake of salvation (cf. Isaiah 42:6; 45:21, 22; 49:6; 52:10; 60:1–3; Psalms 22:27; 65:5; 98:2, 3), but it did not reveal that the Messiah would actually live in each member of His redeemed church, made up mostly of Gentiles.
That believers, both Jew and Gentile, now possess the surpassing riches of the indwelling Christ is the glorious revealed mystery (John 14:23; Romans 8:9, 10; Galatians 2:20; Ephesians. 1:7, 17, 18; 3:8–10, 16–19). the hope of glory. The indwelling Spirit of Christ is the guarantee to each believer of future glory (Romans 8:11; Ephesians 1:13, 14; 1 Peter 1:3, 4).
MacArthur, J. J. (1997). The MacArthur Study Bible (electronic ed.) (Col 1:27). Nashville: Word Pub. www.gty.org