Paul is assuming that his social-theological explanation of chapters 1-14 ought to make a difference in lives of the two social groups, especially in how to interact and treat one another. It appears that in chapter 15-16, Paull wants to tie it all together so to speak, to ensure that the the two socially distanced groups begin to bridge the gap between themselves and learn to live for the glory of God.
So, he uses chapter 15-16 to mirror what he has already been saying throughout his entire letter, especially the first chapter.
Paul's ultimate goal, throughout the entirety of his letter, comes into clear focus in his prayer in 15:5,
“May the God of endurance and encouragement grant you to live in such harmony with one another, in accord with Christ Jesus, that together you may with one voice glorify the God and Father of our Lord Jesus Christ. Therefore, welcome one another as Christ has welcomed you, for the glory of God.”
Notice the double use of the word “glory” in this prayer. God’s glory is Paul’s ultimate purpose!
He has already told his readers that “He rejoices in hope of the glory of God” being someday displayed to the entire world. (5:2) And he has also already told his readers of God’s plan that God prepared beforehand, which is to make known His power and riches of His glory by using the Jews and Gentiles as his vessels for destruction and mercy (9:24) to bring the Gentiles to the obedience of faith.
The problem, as Paul has already mentioned to his Jewish readers, is that the reason that they are in the condition they are in is because they have devalued the glory of God. Paul has already confronted his readers saying that they have “exchanged the glory of the immortal God for images resembling mortal man and birds and animals” (1:23) He has already told his readers that “all have sinned and fall short of God’s glory.” (3:23) He has already told his readers that God intends that, through whatever means, that His truth will abound to his glory.” (3:7) which is the reason Paul writes to the Romans; to help them understand the truth of the gospel so god's glory may abound.
Even in his closing remarks to his letter (16:25-26), Paul is concerned about God’s glory. He offers up a final prayer after all his greetings, saying,
“Now to him who is able to strengthen you according to my gospel and the preaching of Jesus Christ, according to the revelation of the mystery that was kept secret for long ages but has now been disclosed and through the prophetic writings has been made known to all nations, according to the command of the eternal God, to bring about the obedience of faith— to the only wise God be glory forevermore through Jesus Christ! Amen.” (16:25-27)
It is this final prayer that is so striking. Could it be that Paul is attempting one last effort at securing his premise to the rest of the letter by reinforcing it here? Comparing this closing paragraph with Pauls opening paragraph in 1:1-6, makes one wonder at his intentionality in constructing his letter to best communicate with clarity. Does it not look intentional that Romans ends as Romans begins? Notice the underlined words, phrases and ideas from the last paragraph of his letter (above) that are repeated from the first paragraph of his letter (below).
Paul, a servant of Christ Jesus, called to be an apostle, set apart for the gospel of God, which he promised beforehand through his prophets in the holy Scriptures, 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. (1:1-6)
In addition to these repetitive details, Paul also started his letter by letting his readers know that he wanted to “strengthen them” by imparting to them some spiritual gift (1:11). He ends his letter by acknowledging that God is the one who can do that strengthening.
Paul started his letter by referring to him “not being ashamed of the gospel” (1:16) for which “he was set apart to preach.” He ends his letter here referring to his preaching of Jesus in his gospel.
Paul started his letter with a reference to the gospel being promised beforehand (1:2). He ends his letter with a reference to the mystery of the gospel kept secret for long ages.
Paul started his letter with a reference to the gospel message being declared or specified (1:4). He ends his letter with a reference to the gospel being disclosed.
Paul started his letter with a reference to the gospel being promised through his prophets in the holy Scriptures (1:2). He ends his letter with a reference to the gospel being disclosed through the prophetic writings.
Paul started his letter with a reference to Jesus being declared the Son of God for the sake of his name among all the nations. He ends his letter saying that the gospel has been disclosed, and through the prophetic writings has been made known to all nations.
Paul started his letter by describing that the gospel is intended to bring about the obedience of faith among the nations. He ends his letter by saying that the gospel was intended to bring all nations to the obedience of faith.
So, is it not clear that Paul is attempting to connect the end of his letter with the beginning of his letter? What’s more, by default in doing so, he connects his socio-theological explanation to the plethora of commands in the previous section (chapters 12-14). It appears that in the mind of Paul, knowing the truth about God ought to lead to living that truth so that God’s glory abounds.
Paul is convinced that in order to live honorably, we must know the truth of his gospel, present ourselves to God as a living sacrifice just as Jesus did, and treat one another with honorable acts, contrary to the way Paul describes his readers in the first chapter.
To convince his readers of this point, he has spent the first 8 chapters explaining that God does not show partiality, that all have sinned and fall short of the glory of God and that all are in need of justification that only can come about by faith in the death and resurrection of Jesus that saves a person from the wrath of God.
This salvific faith is available to everyone, historically to the Jews first, and then the Gentiles. The Jewish nation is currently experiencing a partial hardening so that the Gentiles have their time of salvation. The social-theological context of the Jewish/Gentile relationships is explained in detail in chapters 9-11.
For the remainder of the letter, it has been Pauls attempt to help his readers honor God’s plan as they honor one another, and live as if they have died with Christ themselves.
The connection that Paul makes with the beginning and ending of his letter only serves to emphasize the importance he puts on living in harmony with one another, honoring and respecting those in opposing groups.