WHAT IS "THE MYSTERY"?
In Paul's letters, "mystery" does not mean something mysterious or puzzling. It means something that was hidden in God's heart from eternity, concealed throughout the ages, and has now been unveiled in Christ and made known to His people. It is God's secret — now told.
"To me, who am less than the least of all the saints, this grace was given, that I should preach among the Gentiles the unsearchable riches of Christ, and to make all see what is the fellowship of the mystery."
EPHESIANS 3:8–9 (NKJV)