If God really exists, why does he not just reveal himself to everyone and get the question of his existence over and done with? This is a bit of a mystery, made all the more mysterious by the claim that God already has revealed himself to everyone, but most people have chosen to reject him and to devise other explanations for the things God has done. Paul talks about the mystery in chapter 3 of his letter to the Ephesians:
“I became a servant of the good news according to the gift of God’s grace which was given to me by means of his power. This grace was given to me, the very least of all believers, enabling me to announce the unsearchable riches of Christ to the Gentiles, and to explain to everyone the plan of the mystery which for ages has been kept secret by God, who created all things through Jesus Christ.”
According to Paul, the “mystery” is the good news of Jesus Christ. All people have sinned and fall short of the glory of God, and the only way for anyone to be reconciled to God and live with him forever is for each person’s sin to be paid for and cancelled. Jesus paid for our sins by dying for us on the cross, and the only way we can be reconciled to God is through accepting the free gift of grace given to us by Jesus.