Romans 15:8

For I say that Christ hath been made a minister of the circumcision for the truth of God, that he might confirm the promises given to the fathers, and that the Gentiles might glorify God for his mercy; as it is written, Therefore will I give praise to thee among the Gentiles, And sing unto thy name.

Circumcision ... means the Jews; and the confirmation of "the promises given to the fathers" refers to God's sending, at last, the Messiah, the true "seed" promised to Abraham. Thus, again, the long discussion of the relationship of Jews and Gentiles to God in earlier chapters of Romans came vividly to Paul's mind, suggesting that the problem relating to scruples was related to the long conflict between Jews and Gentiles; and therefore, as a further reinforcement of his commandments here, he returned to the fact of God's purpose of containing both Jews and Gentiles in one body in Christ.

This thought appears also in this comment by Barrett:

The coming of Christ may be viewed in two ways. On the one hand, he came to vindicate God's promises which had been made within Judaism. On the other hand, he came that the Gentiles might, be included with Israel among the people of God. As the Jews glorify God for his faithfulness, so the Gentiles will glorify him for his mercy.[3]

The Old Testament quotation Paul used here is found twice, in 2 Samuel 22:50 and Psalms 18:49, and shows that the Gentiles, the heathen, or nations, as non-Jews were variously described, were certainly included in God's ultimate purpose of redemption, "that he might create in himself of the two one new man, so making peace" (Ephesians 2:15).