Paul reminds the Christians in Rome that Jesus was a servant. He does not say that Jesus became a servant of the Jews although he does say that Jesus served the Jews. We could say that the Jews were the recipients of his service, but that this service was done in response to his Father’s call. What did his service involve?
The answer to that question is that he came to reveal or to display the faithfulness or the truthfulness of God. This unfolding of God’s reliability was seen in two ways: first, Jesus confirmed the promises given to the patriarchs and, second, he made it possible for the Gentiles, who were outsiders, to glorify God for his mercy.
The patriarchs were Abraham, Isaac and Jacob, the forefathers of the Israelites. God had promised to them that through their line the Messiah would come into the world. So Paul is reminding his readers that those promises made to the patriarchs had been fulfilled.
Yet the Old Testament said that there was more to God’s plan that what he would do for Israel in sending the Messiah. In addition, mercy would be extended to the Gentiles and many of them would thereby praise God for sending Jesus. The unwritten point here is that Jesus became a Jew in order to obey God’s law perfectly and so offer himself as a payment for the sins of both Jews and Gentiles. The fact that he did so made it possible for God to show mercy. It is only because of the cross of Jesus that God can forgive sinners for their rebellion and then show other aspects of mercy to them as well such as welcoming them into his family.
What had this to do with the disputes between the strong and the weak mentioned earlier in the chapter by Paul? They were finding it difficult to accept one another because of ethnic practices. Yet the fact was that neither of them were acceptable to God without Jesus and what he had done with their sins. He had died to bring Jews and Gentiles together. And the strong and the weak Christians should remember that was the case.