In Romans 15, Paul continues his exhortation to the church at Rome to bear with one another with charity, the “strong” not lording their freedom over the “weak” in conscience, and concludes by insisting, “Now I say that Jesus Christ was a minister of the circumcision for the truth of God, to confirm the promises made unto the fathers: And that the Gentiles might glorify God for his mercy” (Rom 15:8-9a). That is: don’t think that Christ came more for the Jews than for the Gentiles; he came to confirm God’s promises to Israel, and to extend the mercy of God to the nations.
But then he immediately goes on to offer four quotations from the OT, justifying his claim that the Gentiles would in fact become the objects of the LORD’s mercy; apparently, God has made promises to the nations, as well as to Israel. A few notes about this: the four quotations come from Deuteronomy, the Psalms (2x), and Isaiah; for Paul, “the Law, the Prophets, and the Psalms” testify to the ingathered church (cf. Lk 24:44). And his ordering of the quotations has a neat, chiastic structure:
A. “For this cause I will confess to thee among the Gentiles, and sing unto thy name” (Rom 15:9, Ps 18:49). The next verse in the Psalm is surely significant here, perhaps being evoked metapleptically: “Great deliverance giveth he to his king; and sheweth mercy to his anointed, to David, and to his seed for evermore” (Ps 18:50). The psalm links the ingathering of the nations to the Davidic kingship.
B. “Rejoice, ye Gentiles, with his people” (Rom 15:10, Deut 32:43).
B’. “Praise the Lord, all ye Gentiles; and laud him, all ye people” (Rom 15:11, Ps 117:1). Two passages that depict the Gentiles praising the LORD side-by-side with Israel.
A’. “Esaias saith, There shall be a root of Jesse, and he that shall rise to reign over the Gentiles; in him shall the Gentiles trust” (Rom 15:12, Isa 11:10 [LXX]). As in the first quotation/metalepsis, the ingathering of the nations is linked to the rise of the anointed Davidic king.
The burden of these quotations is to show that the plan all along — from Deuteronomy through Isaiah — was for the Gentiles to be gathered into the LORD’s people precisely as Gentiles. These are really important verses for underwriting Paul’s Torah-free mission to the Gentiles. But then, that raises a question: why are we seeing these quotations for the first time at the end of Romans (earlier chapters of which deal extensively with Gentile-inclusion cf. Rom 3-4, Rom 9-11), which is itself one of Paul’s last letters? Why didn’t Paul use these proof-texts to steamroll the Teachers in Galatians?