Col 1:26 What is the mystery Paul refers to here?

The mystery Paul refers to here is Christ indwelling believers, the hope of glory (CP V27 with Jn 14:23; Ro 8:8-11; 1Cor 3:16; 6:19; 2Cor 6:16; Ga 2:20; 4:6; Col 3:11).

The indwelling spirit of Christ is the believer's guarantee of future glory (CP Ro 8:11, 22-23; 2Cor 1:21-22; 5:5; Eph 1:13-14; 1Pe 1:3-4).

The Old Testament predicted that the Gentiles would partake of the salvation benefits in Christ along with the Jews, but it did not reveal that Christ would actually indwell them all (CP Psa 22:27; 65:5; 98:2-3; Isa 42:6; 45:21-22; 49:6; 52:10; 60:1-3).