Romans 15:9 - Matthew Poole's English Annotations on the Holy Bible

Here he proves the second part, that Christ hath also received the Gentiles. There is a plain ellipsis in the words; this is understood, that there were promises made of or to the Gentiles, and Christ came to confirm them also. The sum of these promises was this, that the Gentiles should glorify God for his mercy. Some have observed how the truth of God is spoken of in the foregoing verse with respect to the Jews, and the mercy of God with respect to the Gentiles; not that the one was without the other; for the salvation of the Jews, as it was of truth, so of mercy also, Micah 7:20; and the vocation of the Gentiles, as it was of mercy, so also of truth; for there were many promises of God concerning that matter, but mercy is predicated of the Gentiles, because that attribute of God appeared more eminently in their conversion and calling. You had the like distribution and difference in Romans 4:25, Romans 10:10. As it is written: because the Jews were hardly persuaded of the mercy of God to the Gentiles, therefore he proves it by divers Scripture testimonies. This first is taken out of Psalms 18:49: See Poole on "Psalms 18:49". David speaks this in the person of Christ. In the Psalm it is: I will give thanks to thee; but here, according to the LXX., I will confess to thee, or celebrate thee among the Gentiles. They then are received to mercy, forasmuch as it was foretold they should celebrate or praise God for his mercy.