I have never once expressed that men are saved any other way than by grace through faith, so your assumption by what I understand as the mystery is without merit.
Abraham was a Gentile when he was justified by faith.
Becuase they were looking forward to the promise. So?
God made a covenant with Abraham.
Those who believe God
How do you explain stating: " . . . a mystery of good news to the Gentile through Christ,
which had not been revealed until the proper time?"
Are you saying that O.T. believers exhibited and lived according to blind faith?
I contend the gospel remained unknown and the promises were not believed, because of God's reprobation of many souls (both Jew or Gentile). Isaiah 6:9-10, John 12:37-40, II Corinthians 4:3, Romans 11:7-10, etc.
Only those who receive the grace of God and are gifted with faith to believe, can comprehend the heavenly kingdom promises. I Corinthians 2:14
Knowledge given of this kingdom is the good news, and it is the assurance of redemption worked for all kinds of sinners. National Israel (reprobates) mistakenly thought they alone were the chosen people of God, and at large, shunned non-Jews.
The latter
reconciliation worked between Jews and Gentiles through this saving enlightenment of Truth to the church at Ephesus,
is the "mystery" referred to by Paul in that epistle.
But only Christians grasp this part of the good news, because they alone are the recipients and benefactors of the gospel and anointing of the Holy Spirit, to begin with.
Reprobates still do not understand this "mystery" that
both a Jewish remnant and a remnant, called out of all the nations of the world, make up the invisible, heavenly (church) Kingdom of God. See Rev. 5:9 and 7:9.
Reprobates still don't get it . . . that is what defines them as being "reprobate." God does not open their understanding to these gospel truths.