Does the inclusion of the Gentiles as believers under the New Covenant overturn the Old Covenant promises to Israel?

No, for in Romans 9:25 the calling of the Gentiles is revealed as a calling that is not based on genes but on the God’s own promise that had already been proclaimed through the prophets: As indeed he says in Hosea, ‘Those who were not my people I will call my people, and her who was not beloved I will call beloved. In other words, just as God raised the younger Isaac above Ishmael and the younger Jacob above Esau, so he now raises the younger people of Christ above the old unbelieving Jewish people: And in the very place where it was said to them, You are not my people, there they will be called sons of the living God (Romans 9:26).

The reverse side of the exaltation of many believers from among the Gentiles is, however, the wrath on many Jews who reject the gospel. God’s grace is decisive—even concerning Israel. As the prophet Isaiah once exclaimed, Though the number of the sons of Israel be as the sand of the sea, only a remnant of them will be saved, for the Lord will carry out his sentence upon the earth fully and without delay (Romans 9:27–28). Here we see again what Paul already noted in Romans 9:6: God’s Word never changes and his promise never fails.

Finally in Romans 9:29 the apostle quotes yet another word from the prophet Isaiah which also confirms this: If the Lord of hosts had not left us offspring, we would have been like Sodom and become like Gomorrah (Romans 9:29). This quote quite possibly also served to echo a statement on this matter made by Jesus himself in Matthew 11:23–24: And you, Capernaum, will you be exalted to heaven? You will be brought down to Hades. For if the mighty works done in you had been done in Sodom, it would have remained until this day. But I tell you that it will be more tolerable on the day of judgment for the land of Sodom than for you.¹