Jesus’ service on earth, while conducted among his own Jewish people, always had all the nations of the earth in mind. He also came to earth in order that the Gentiles might glorify God for his mercy. As it is written, Therefore I will praise you among the Gentiles, and sing to your name
(Romans 15:9). The spreading of the Gospel among the Gentiles was precisely what was promised to the patriarchs in Genesis, and as such the conversion of the Gentiles constitutes the very confirmation of those promises. This is why it is followed-up by several quotations from the Old Testament which proclaims this promise (Romans 15:9–12).
Romans 15:9 and Romans 15:8 are therefore more closely interconnected than some translations seem to suggest, and the transition from Romans 15:8 to Romans 15:9 can best be translated as follows: to confirm the promises given to the patriarchs, that the Gentiles should glorify God for his mercy.
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8 For I tell you that Christ became a servant to the circumcised to show God’s truthfulness, in order to confirm the promises given to the patriarchs,