Albert Barnes Commentary Romans 15:9

"and that the Gentiles might glorify God for his mercy; as it is written, Therefore will I give praise unto thee among the Gentiles, And sing unto thy name." — Romans 15:9 (ASV)

And that the Gentiles, etc. The benefits of the gospel were not to be confined to the Jews; and as God designed that those benefits should be extended to the Gentiles, so the Jewish converts ought to be willing to admit them, and treat them as brethren. That God did design this, the apostle proceeds to show.

Might glorify God. Might praise, or give thanks to God. This implies that the favor shown to them was a great favor.

For his mercy. Greek, On account of the mercy shown to them.

As it is written. Psalms 18:49. The expression there is one of David's. He says that he will praise God for his mercies among the heathen, or when surrounded by the heathen; or that he would confess and acknowledge the mercies of God to him, as we should say, to all the world. The apostle, however, uses it in this sense, that the Gentiles would participate with the Jew in offering praise to God, or that they would be united. This does not appear to have been the original design of David in the psalm, but the words express the idea of the apostle.

And sing, etc. Celebrate your praise. This supposes that benefits would be conferred on them, for which they would celebrate his goodness.