John Chrysostom

"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.

Homily on Romans 16 ↗

Theophylact of Ohrid

The Jews could say: you, Paul, are deceiving us by saying this. Therefore he brings forward Hosea, who cries out and says: "I will call those who were not My people, My people." Who then was "not His people"? The Gentiles. And who was "not beloved"? The Gentile Church. Yet the Gentiles became a people and beloved.

Commentary on Romans ↗

Irenaeus

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 …

Against Heresies Book III ↗

Thomas Aquinas

Then he proves the proposition, when he says as in Hosea he says: first, with respect to the gentiles; second, the Jews, at and Isaiah cried out. In regard to the first he cites two texts from Hosea speaking for the gentiles: the first of these promises them God's gifts; the second, divine sonship, at and it shall be in the place where it was said unto them. …

Commentary on Romans ↗