Charles Spurgeon Commentary Romans 15:9-12

"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. And again he saith, Rejoice, ye Gentiles, with his people. And again, Praise the Lord, all ye Gentiles; And let all the peoples praise him. And again, Isaiah saith, There shall be the root of Jesse, And he that ariseth to rule over the Gentiles; On him shall the Gentiles hope." — Romans 15:9-12 (ASV)

There were plain indications in the Old Testament that the blessing was meant for the Gentiles as well as for the Jews; but still, it was made known to the Jews first, and we must never forget that.