Romans 9:25

The gist of the entire argument leads to this conclusion. Paul taught that we do good by the mercy of God and that the Jews who had received the gospel should not glory in their works, thinking that they had deserved this and not wanting it to be given to the Gentiles. In Paul's mind, the Jews should cease from such pride and understand that if we are called to faith not through our own works but by the mercy of God and if it is given to those who believe to do good, then they should not begrudge the Gentiles this mercy as if it had been given to the Jews on the ground of prior merit, which is nothing.

It is clear that this was said about the Gentiles, who once were not God's people, but afterward, to the chagrin of the Jews, received mercy and are called God's people. Once they were not loved, but when the Jews fell away they were adopted …

For Matthew the apostle-knowing, as one and the same God, Him who had given promise to Abraham, that He would make his seed as the stars of heaven, and Him who, by His Son Christ Jesus, has called us to the knowledge of Himself, from the worship of stones, so that those who were not a people were made a people, and she beloved who was not beloved-declares that John, when preparing the way for Christ, said to those who were boasting of their relationship [to Abraham] according to the flesh, but who had their mind tinged and stuffed with all manner of evil, preaching that repentance which should call them back from their evil doings, said, "O generation of vipers, who hath shown you to flee from the wrath to come? Bring forth therefore fruit meet for repentance. And think not to say within yourselves, We have Abraham [to our] father: for I say unto you, that God is able of these stones to raise up children unto Abraham." He preached to them, therefore, the repentance from wickedness, but he did not declare to them another God, besides Him who made the promise to Abraham...

But this is what we ought to attend to more sharply: that those others also obtained mercy, and were called the people of God, and were beloved of God; but because they did not know how to keep the grace they had received, it is said to them: "In the things wherein the dwelling of Israel committed adultery, I put her away, and gave her the bill of divorce into her hands"; and again elsewhere: "You have become a surfeit to me, I will no longer forgive your sins." And through Jeremiah the Lord says: "My inheritance has become to me like the cave of a hyena." Lest perhaps we too, who were not the people of God, but through the riches of his glory he called us his people, and who were not beloved, but have been made beloved, and sons of the living God — if we do not walk as sons of light and sons of God, if we do not so act as the people of God, "that men, seeing our good works, may glorify our Father who is in heaven" — it is to be feared that we may incur that saying of the Apostle in which he says: "For if God did not spare the natural branches, neither will he spare you. As he says in Hosea: I will call that which was not my people, my people; and her that was not beloved, beloved; and her that had not obtained mercy, her that had obtained mercy. And it shall be: In the place where it was said to them, You are not my people, there they shall be called sons of the living God." This testimony, indeed, which the Apostle takes from Hosea, so far as pertains to the passage of the prophet itself, seems to be spoken not about the Gentiles, but about the people of Israel themselves. […]

"I will call them My people, which were not My people; and her beloved, which was not beloved." Here to prevent their saying, that you are deceiving us here with specious reasoning, he calls Hosea to witness, who crieth and saith, "I will call them My people, who were not My people." Who then are the not-people? Plainly, the Gentiles. And who the not-beloved? The same again. However, he says, that they shall become at once people, and beloved, and sons of God.