In the first chapters of Romans, Paul seems to be concerned with making at least two arguments: 1) the gospel is available to all who believe, both Jewish person and Gentile, and 2) the gospel he preaches (justification by faith alone) is in full continuity with the salvation revealed in the Old Testament Scriptures.
Consider verse 2, which says that the gospel was “promised beforehand through his prophets in the holy Scriptures.” Then he describes the gospel in vv. 3-5 following,
“concerning his Son, who was descended from David according to the flesh and was declared to be the Son of God in power according to the Spirit of holiness by his resurrection from the dead, Jesus Christ our Lord, through whom we have received grace and apostleship to bring about the obedience of faith for the sake of his name among all the nations, including you who are called to belong to Jesus Christ,”
The blessed Apostle continues he wants to go to Rome so that he can preach the gospel to the Gentiles there, “in order that I may reap some harvest among you as well as among the rest of the Gentiles. I am under obligation both to Greeks and to barbarians, both to the wise and to the foolish.” He earnestly desired to preach the gospel to the Gentiles in Rome. Then he continues his line of reasoning,
ου γαρ επαισχυνομαι το ευαγγελιον του χριστου δυναμις γαρ θεου εστιν εις σωτηριαν παντι τω πιστευοντι ιουδαιω τε πρωτον και ελληνι δικαιοσυνη γαρ θεου εν αυτω αποκαλυπτεται εκ πιστεως εις πιστιν καθως γεγραπται ο δε δικαιος εκ πιστεως ζησεται. (For I am not ashamed of the gospel, for it is the power of God for salvation to everyone who believes, to the Jew first and also to the Greek. For in it the righteousness of God is revealed from faith for faith, as it is written, “The righteous shall live by faith.”)
Thus when Paul says “I am not ashamed of the gospel of Christ,” he does not mean shame in the sense that we speak of it today. The emphasis is in the “to everyone who believes”; He is confident that it is able to work in all who believe, and it does not matter if they are Jew or Greek. In fact, Paul says “to the Jew first and also to the Greek.” I think he means here that the Jewish people, based on their status as God’s chosen people, first heard the good news concerning the coming Messiah, first believed, and were first justified. This is a chronological first, not a first of priority (though I am not yet willing to die on that hill). The Jewish people first heard the gospel of salvation through the coming Christ. It worked in them, now Paul is confident that God will work through the good news in the Gentiles as well (he uses “Greek” as a synonym for “the nations” (v 5). I think he is speaking chronologically here foreshadowing what he will say in Romans 9:4-5,
“They are Israelites, and to them belong the adoption, the glory, the covenants, the giving of the law, the worship, and the promises. To them belong the patriarchs, and from their race, according to the flesh, is the Christ who is God over all, blessed forever. Amen.”
Then in the eleventh chapter, Paul says,
“So I ask, did they stumble in order that they might fall? By no means! Rather through their trespass salvation has come to the Gentiles, so as to make Israel jealous. Now if their trespass means riches for the world, and if their failure means riches for the Gentiles, how much more will their full inclusion mean!”
It is on this basis that Paul admonishes the Gentile believers in vv 17-18,
“But if some of the branches were broken off, and you, although a wild olive shoot, were grafted in among the others and now share in the nourishing root of the olive tree, do not be arrogant toward the branches.”
Therefore in a real sense, the Jewish people were the first to have the gospel working in them; now the Gentiles have it. By the way, I believe that there is a sense in which the Jewish people have a “priority” over the Gentiles. But I do not believe that is the import of 1:16-17.
Paul continues that he knows all of this because in the gospel “the righteousness of God has been revealed εκ πιστεως εις πιστιν (from faith to faith).” This is, of course, a difficult phrase to interpret. 2 Corinthians 2:16 has a similar construction when it says (starting in v 15), “For we are the aroma of Christ to God among those who are being saved and among those who are perishing, to one a fragrance from death to death [εκ θανατου εις θανατον], to the other a fragrance from life to life [εκ ζωης εις ζωην]. Who is sufficient for these things?” I think that we can look at this passage and see a kind of transfer of death and life, from the believers to unregenerate and from the believers to those who believe. When Paul says in Romans 1 that the righteousness of God has been revealed in the gospel “εκ πιστεως εις πιστιν,” he means “from their faith to our faith.” We have seen the justifying work of the gospel throughout all time from the faith of the Jews beforehand to the faith of the Gentiles at the present time. The gospel has remained the same throughout: God justifying the ungodly.
Therefore he can say later in Romans 3:21-26 that all can be saved. The emphasis here again is that anyone, including the Gentiles, can be saved by faith. How is this possible? He answers, “Since all have sinned and come short of the glory of God.” Since all have sinned, all can be “justified by his grace as a gift.” God justified the Jewish people by faith, now he is justifying the Gentiles by faith. Notice that Paul even points out that the sacrifice of Christ provided righteousness for the sins God had “passed over” in “divine forebearance,” and righteousness for the sins “at the present time.” It is in this that God shows himself to be the “just and the justifier of the one who has faith in Jesus.” Whether that faith is forward-looking (the Jewish people) or backward-looking (and forward-looking for us), when we believe in Jesus, we are justified. This is the way it has always been. The importance of the continuity of this faith is part of reason Paul shows that this “justification by faith alone” gospel he is preaching was the same thing believed by Abraham and David (ch. 4). And it is for this reason the Apostle says in Romans 10:11-13,
“For with the heart one believes and is justified, and with the mouth one confesses and is saved. For the Scripture says, “Everyone who believes in him will not be put to shame.” For there is no distinction between Jew and Greek; the same Lord is Lord of all, bestowing his riches on all who call on him. For “everyone who calls on the name of the Lord will be saved.”
All English Scripture cited is from the English Standard Version.