What does Colossians 1:26 mean?

Paul calls the word of God or the gospel a "mystery," but a mystery which has now been revealed. Paul spoke elsewhere about the gospel as a mystery, stating, "according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages" (Romans 16:25). Before Jesus came to earth, no one knew exactly how God planned to fulfill all of His many prophecies regarding the Jewish Messiah. The prophets who wrote about the future only vaguely understood the message they were given (Ephesians 3:4–5). The complete story was only revealed after Christ came in the flesh.

Even during Christ's ministry, Paul was an unbeliever. Jesus revealed Himself to Paul on the road to Damascus (Acts 9), and "the mystery was made known to [him] by revelation" (Ephesians 3:3). Paul asked the Ephesians to pray "also for me, that words may be given to me in opening my mouth boldly to proclaim the mystery of the gospel" (Ephesians 6:19)." In Colossians, Paul will again refer to "mystery" in the next verse, as well as Colossians 2:2 and Colossians 4:3.