The mystery is clearly the gospel or the good news about Jesus if you read Colossians 1:26 in context. There are certain passages in the Old Testament where God preaches the gospel in advance or prophecies about the gospel, but they are not clear enough which is why the Jews missed their own Messiah. God did not reveal the gospel in its fullness in advance because if they had realized who Jesus was, they would not have crucified him. ◄ 1 Corinthians 2:8