What does Romans 15:9 mean?
Paul is describing the reasons Christ came to earth. First, He was sent to become a servant to the Jewish people. He was, in fact, the means by which God kept all the promises He had made to Abraham and the patriarchs of Israel.Now Paul says that Christ also became a servant to the Jews for another reason: so that the Gentiles would glorify God for His mercy. When the Israelites, for the most part, refused to come to God through faith in Christ, God offered the gift of His grace through faith in Christ to the rest of the world (Romans 11:11–12). That's how Jesus' mission to serve the Jews resulted in so many Gentiles giving glory to God for His mercy in forgiving their sins and sharing His glory with them.
Paul offers several quotations from the Old Testament Scriptures to show that this was always God's plan. He begins with the quote in this verse, most likely from Psalm 18:49. It's also possible Paul is citing 2 Samuel 22:50. The picture Paul uses this verse to construct is of David or the Jewish people praising God alongside the Gentiles.