References: Romans 11
What was a mystery concerning the salvation of Gentiles through the same Jewish Messiah as the Jews had not been previously been made known in the Old Testament. God, although not yet having revealed the details of His plan of salvation for Gentiles, had known the plan before the foundation of the world. Details of the mystery being revealed include the partial hardening of the Jews and how previous revelation fits together with the new revelation without changing the previous. The Old Testament had many details, but not enough to fit together a complete overall picture.