The grace of God was promised to Abraham long before Paul was born.
The grace of God was in God’s mind way before Abraham, way before Adam, even. It was hid in God from before the foundation of the world, but not made known until Paul. Prophecy and mystery, editor. A kingdom of believers and a body of believers. Law v. grace. Israel and the Gentiles. On God’s timeline. That HE chose to reveal to man gradually, not all at once. Otherwise, why would Abraham have believed God and begin to sacrifice his son? Why wouldn’t he have simply said, I believe that Christ will die for my sins, be buried and rise again the third day for my justification. Why wouldn’t Moses say the same thing? And David? And Matthew, Mark, Luke and John? And, by the way, Christ Himself, before He died on the cross? What did Christ tell His apostles (Peter and the 11) to preach? The gospel of the kingdom. Do you know what that is and where it is found?