Let’s start with a little history. Paul had never been to Rome, nor does he speak of any other apostle as having been there or founding that congregation. It appears likely to have been founded by Christian travelers, trades people most likely.
The Roman church would likely have had a Jewish core (as was generally true of churches in Gentile lands) along with converted Gentile God fearers (uncircumcised Gentiles who’d come to believe in the God of the Jews before Christianity arrived), as well as other Gentiles.
Around 49 AD, the Emperor Claudius expelled all Jews from Rome, which would have converted the church there to an all-Gentile church and deprived the church of most of its ablest scholars of the scriptures — as the NT was largely not yet written and the few extant letters of Paul were likely not yet copied and available to distant churches. Likely the leadership of the church had been primarily Jewish, as the Jews would have known the OT scriptures so well.
Later, the Jews were allowed back into Rome, and Romans clearly shows that Jews were present among the membership when it was written around 54 AD. But you can imagine the trauma all this would have brought to the church. The Jews had been the most prominent members, counted on for their expertise in the OT scriptures. While they were gone, Gentile members were forced into positions of leadership. When the Jews returned, they found a church that had Gentile leadership and teachers. While the returning Jews were beloved and accepted, it was not the same church. The social dynamics had changed. Tensions would surely have arisen.
Therefore, Romans concludes with chapters 14-15, which deal with the practical problems of how Jews and Gentiles get along in the same congregation. Rom 12 addresses leadership based on spiritual gifts rather than, say, seniority or who held the church together when the Jews were forced to leave or who held office before the Jews were forced to leave. It’s about giftedness, not entitlement.
The second half of Rom 12 deals with Christian ethics, largely in terms of how to love each other properly. It’s about how to get along. Chapter 13 is about love fulfilling the law.
Rom 9 – 11 deals with the tragedy that most Jews had failed to accept Jesus, resulting in the blessing of Gentiles’ coming into the Kingdom. Does this mean the Kingdom is Gentile and not Jewish? or does the Jewishness of Jesus make the Kingdom Jewish and not Gentile? Paul deals with how the Jews and Gentiles should see each other through the lens of OT prophecy, and he explains how what is happening (Jews rejecting Jesus, Gentiles accepting Jesus) is in fulfillment of prophecy.
Rom 6 – 8 deals with the rationale for living as a Christian should, culminating in a powerful lesson on the Spirit in the first half of chapter 8, followed by an even more powerful declaration regarding the glories of the fulfillment of God’s destiny. But Paul always thinks in terms of OT prophecy, and so he would have seen God’s destiny in terms of the fulfillment of prophecy — as he explains in detail in chapters 9 – 11.
God’s “foreknowledge” is demonstrated also by his prophecies, proving that God knew the Kingdom would become heavily Gentile, with most Jews rejecting Jesus as Messiah. The Roman church was among the earliest churches in which Gentiles became the dominate ethnic group — and this forced some serious reflection on how the Kingdom of God, long promised by the Jewish prophets as marking the end of the Exile of the Jews that began with the Babylonian Captivity, could result in a church dominated by Gentiles.
Hence, Rom 9 – 11 is no mere parenthetical. It may well be the climax of Paul’s elaborate argument beginning in chapter 1. It may be the singular purpose of the epistle.
Chapter 5 deals with the role of Torah in salvation history. Chapter 4 speaks to the role of God’s covenant with Abraham in salvation history. Chapter 3 speaks to salvation by faith rather than Torah.
Chapters 1 and 2 speak to the fact that both Jews and Gentiles deserve damnation because both are accountable for enough sin to damn themselves. Therefore, both groups need salvation.
Throughout Romans, Paul speaks of Jews and Gentiles. “To the Jew first, but also to the Greek” is a recurring theme.
(Rom. 1:16 ESV) 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.
(Rom 2:9-10 ESV) There will be tribulation and distress for every human being who does evil, the Jew first and also the Greek, 10 but glory and honor and peace for everyone who does good, the Jew first and also the Greek.
(Rom. 3:9 ESV) What then? Are we Jews any better off? No, not at all. For we have already charged that all, both Jews and Greeks, are under sin,
(Rom. 3:29 ESV) Or is God the God of Jews only? Is he not the God of Gentiles also? Yes, of Gentiles also,
(Rom. 9:24 ESV) even us whom he has called, not from the Jews only but also from the Gentiles?
(Rom. 10:12 ESV) For there is no distinction between Jew and Greek; for the same Lord is Lord of all, bestowing his riches on all who call on him.
Plainly, Paul is addressing Jew/Gentile relationships within the church. In fact, he begins by emphasizing the Jews’ priority — “to the Jews first” — but ends up declaring that “there is no distinction between Jew and Greek.” The Jews were once blessed with unspeakable gifts from God, but now God is “bestowing his riches on all who call on him” (Rom 10:12).
Therefore, this is not a book for or against Calvinism or Pelagianism or Augustinian thought. It’s about how to fit the facts on the ground into the dashed expectations of the Jews — who expected their Messiah to bring about the Kingdom, but a Kingdom that would bring salvation to the Jews — at a time when most Jews rejected the gospel of Jesus of Nazareth and Gentiles were becoming the larger ethnic group, even taking key positions of leadership.
And answering that question involves working through the prophecies (as Paul does in chapters 9 – 11) but also the covenants (chapters 4 – 5). Paul needed to strip away countless false understandings of the gospel in order to present a better understanding that takes into account a better understanding of the covenants and the prophecies. And in so doing, Paul reveals to the generations that follow countless truths about the gospel that we’d otherwise not know.