8.9 Colossians -- The Mystery
God has revealed to believers the mystery hidden for ages -- which is that the Gentiles are included in God's worldwide plan of salvation.
...the mystery that has been kept hidden for ages and generations, but is now disclosed to the saints. To them God has chosen to make known among the Gentiles the glorious riches of this mystery, which is Christ in you, the hope of glory (Col 1:26-27).
"The mystery is the fact that Christ indwells Gentiles, for it had not been previously revealed that the Gentiles would be admitted to the church on equal terms with Israel" (NIV, p 2298 Footnote).
Paul covers this theme at more length in Ephesians 3 (see sections 2.4.14and 2.4.15 of this study).
Paul wants the Colossians to realize their value to God, and to understand the privileges He has given them. He doesn't want them to jeopardize their redemption by succumbing to the wisdom of the world. He is telling them that in Christ, and only in Him, they have all the blessings of God. They cannot add to them by superstitious beliefs in angels or performing religious rituals.