In his letter to the Colossians the Apostle Paul refers to the gospel message as a mystery:
the mystery that has been kept hidden for ages and generations, but is now disclosed to the Lord’s people (Colossians 1:26 NIV)
Before Jesus came on the scene believing Jews knew that God was a Holy God who was uncontaminated by sin. But they also knew that God was a loving and merciful God that forgave sin. How could these two principles were reconciled? The Old Testament Jews did not really know the answer to this, which is why Paul refers to it as “the mystery that has been kept hidden for ages and generations.” But today we know the answer to the riddle: it is the blood of Christ. It is through Jesus’ sacrifice on the cross for our sins. Now that the truth has been revealed, we are responsible for believing it. Believing in just the Old Testament is not good enough any more. We must believe in Jesus who died on the cross for our sins.
This great truth has been disclosed to “the Lord’s people.” If you believe in it you are one of God’s people, if you don’t you are not.