I disagree with the conclusions you draw as to the mystery Hid in God.
Paul is quite clearly explaining his ministry to the Gentiles. The Jewish people understood what it was to have a sacrifice offered for their sins. The high priest did it every year. That the great dispensation of Jesus would be offered to the Gentiles, to anyone not of the Chosen People, the Jews, was a very new revelation indeed. The OT and thus, the Messiah, was seen as only for God's Chosen People and not for the Gentiles.
Read the beginning of chapter 11 in Acts. Here Peter recounts why the Gentiles had been given and accepted the Word of God. Read the whole thing verses 1-18 and you will see how God revealed to Peter that the word of God was to save all men, not just Jews.
This happened before Paul's discourse on his commission of God's grace that was given him for the sake of the Gentiles. The great mystery was that God intended for all to participate in and receive His grace, not just His Chosen People.
I know...and Paul was saved in Acts 9. Two chapters before Peter gave his Acts 11 preaching..