Matthew Poole and continuators on Romans 15:9

9. And that the Gentiles might glorify God for his mercy, as it is written , For this cause I will confess to thee among the Gentiles, and sing unto thy name.

Here he proves the second part, that Christ hath also received the Gentiles. There is a plain Elipsis 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 glorifie God for his mercy. Some have observed how the truth of God is spoken of in the foregoing verse with respect to the Iewes, and the mercy of God with respect to the Gentiles; not that the one was without the other; for the Salvation of the Iewes, as it was of Truth, so of Mercy also, Mich. 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 chap. 4.25. and 10.10.

Because the Iewes were hardly perswaded of the Mercy of God to the Gentiles, therefore he proves it by diverse Scripture-Testimonies. This first is taken out of Psal. 18.49. See the Notes there. 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.