Albert Barnes on Colossians 1:26

Even the mystery. To make that mystery fully known. Eph 3:2, and Eph 3:3-9. The great doctrine, that salvation was to be proclaimed to all mankind, Paul says, had been concealed for many generations. Hence it was called a mystery, or a hidden truth.

But now is made manifest to his saints. It was communicated especially to the apostles who were appointed to proclaim it, and through them to all the saints. Paul says that he regarded himself as specially called to make this truth known, as far as possible, to mankind.

(c) "mystery" Eph 3:9 (d) "now is made" 2Ti 1:10