John Calvin 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)

The Gentiles also, etc. This is the second point, which he spends more time proving, because it was not as evident.

The first testimony he quotes is from Psalm 18. This psalm is also recorded in 2 Samuel 22, where, undoubtedly, a prophecy is mentioned concerning the kingdom of Christ. From it, Paul proves the calling of the Gentiles, because it is promised there that a confession to the glory of God would be made among the Gentiles.

For we cannot truly make God known, except among those who hear His praises as they are sung by us. Therefore, for God’s name to be known among the Gentiles, they must be favored with the knowledge of Him and come into communion with His people. For you may observe this everywhere in Scripture: God’s praises cannot be declared except in the assembly of the faithful, who have ears capable of hearing His praise.