As he says also in Hosea,
I will call them my people, who were not my people; and her beloved, who was not beloved.
Read Chapter 9
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 as children and are now loved, so that where once they were not called God’s people, now they are called children of the living God. Commentary on Paul’s Epistles. ...
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. ...
And proclaim in what sense .
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.
And it shall come to pass, that in the place where it was said, This is not a people, there shall they be called the children of the living God." ...