16 For I am not ashamed of the gospel, because it is the power of God for salvation to everyone who believes; the Jew first, and also the Greek.
17 For in the gospel the righteousness of God is revealed by faith and by faith, as it is written: But the just shall live by faith.
(RVR1960)
Paul's missionary practice in each city was to begin by preaching the gospel in the Jewish synagogue and then once the Jews heard him or refused to make him preach to the Gentiles which followed the pattern established by God during his earthly ministry and evangelistic work of the primitive church because Jesus himself said: "I came to his own and they did not receive me, but to those who did I gave the power to become children of God"
Paul was a man with conviction when he converted to Christ, he did it in a sure and firm way, convinced that Jesus was the only one who could change man since he himself had proven it when Jesus touched his heart.
That is why Paul says with conviction that he is not ashamed of the gospel because it is the power of God, he knew it very well, he lived in his own flesh the greatest and most divine transformation that any man can receive on earth. Paul received a transformation that changed his heart by removing one of stone and putting one of flesh truly human and full of love, peace and joy transformed by the Lord Jesus Christ himself.
In addition, Paul gave faith that God was not respectful of persons because the gospel was for everyone because the Lord first came to his own to the Jewish people and then he came to the Gentiles.
Paul also testified that the gospel is by faith and for faith since it is through faith that the righteous live.