A.T. Robertson Commentary Colossians 1:27

"to whom God was pleased 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 (ASV)

God was pleased (ηθελησεν ο θεος). First aorist active indicative of θελω, to will, to wish. "God willed" this change from hidden mystery to manifestation.

To make known (γνωρισα). First aorist active infinitive of γνωριζω (from γινωσκω). Among the Gentiles (εν τοις εθνεσιν). This is the crowning wonder to Paul that God had included the Gentiles in his redemptive grace, "the riches of the glory of this mystery" (το πλουτος της δοξης του μυστηριου τουτου) and that Paul himself has been made the minister of this grace among the Gentiles (Ephesians 3:1–2). He feels the high honour keenly and meets the responsibility humbly.

Which (ο). Grammatical gender (neuter) agreeing with μυστηριου (mystery), supported by A B P Vulg., though ος (who) agreeing with Χριστος in the predicate is read by Aleph C D L. At any rate the idea is simply that the personal aspect of "this mystery" is "Christ in you the hope of glory" (Χριστος εν υμιν η ελπις της δοξης). He is addressing Gentiles, but the idea of εν here is in, not among. It is the personal experience and presence of Christ in the individual life of all believers that Paul has in mind, the indwelling Christ in the heart as in Eph 3:17. He constitutes also the hope of glory for he is the Σεκινα of God. Christ is our hope now (1 Timothy 1:1) and the consummation will come (Romans 8:18).