Romans 11:1

Since Paul has shown that the people of Israel did not believe, now, in order that it should not be thought that he has said that they were all unbelievers, he shows that God has not rejected the inheritance which he promised to the descendants of Abraham. For he would not have promised them a kingdom if he knew that none of them would believe.… By using himself as an example, he shows that the part of Israel which God foreknew would be saved had in fact been saved and that the part which had been consigned to perdition because of its constant unbelief might yet be saved.

Fearing once again that exaggerating the rejection of the Jews might lead to a choice of disobedience, Paul turns to the small remnant of the election—the apostles and their fellow believers.

"I say then, Hath God cast away His people whom He foreknew? God forbid."

And he introduces the form a person would use in doubt, as though taking occasion from what had been said, and after making this alarming statement, by the denial of it he causes the sequel to be allowed with readiness; and what by all the former arguments he had been laboring to show that he makes good here also. What then is this? That even if there be but a few saved, the promise yet stands good. This is why he does not merely say "people," but "people which He foreknew." Then proceeding with the proof that the "people" were not cast off, "For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin."

I, he says, the instructor, the preacher. Now since this seemed contrary to what was said before in the words, "Who hath believed our report?" and, "All the day long have I stretched forth My hands to a disobedient and gainsaying people;" and, "I will provoke you to jealousy by them which are no people;" he was not satisfied with the deprecation, nor with having said, "God forbid," but makes it good by taking it up again and saying, "God hath not cast away His people." But this is not a confirmation, men may say, but an assertion. Observe then the confirmation, both the first, and that which follows it. For the first is that he was himself of that race. But He would not, if on the point of casting them off, have chosen from them him to whom He entrusted all the preaching, and the affairs of the world, and all mysteries, and the whole economy. This then is one proof, but the next, after it, is his saying, that "people whom He foreknew," that is, who He knew clearly were suited to it, and would receive the faith. For three, five, even ten thousand were believers from among them. And so to prevent any from saying, Art thou the people, then? And because thou hast been called, hath the nation been called? he proceeds.

Paul has mentioned not only his first ancestor but also the head of his tribe in order to show that he is not fabricating his claim.

This refers to what Paul said above. Only those Jews who have believed in the Lord will be counted as descendants.

Wisely, Paul does not make the plight of Israel appear worse than it is. Even though he says that the nation is struggling in its blindness, he manages to say something positive at this point.

Paul says that, if God had rejected his people, he would have been one of those rejected as well.

Paul could have supported his statement by referring to the 3, who believed at Jerusalem and to the many thousands spoken of by St. James, not to mention all those Jews of the diaspora who believed the message. But instead he uses himself as an example.

Having called the Jews a disobedient people, he presents himself as doubting, saying: have the promises of God not been fulfilled because the people of Israel became disobedient? No, he answers. God has not rejected His people, "whom He foreknew," that is, whom He knew to be capable of receiving the faith. "For I also am an Israelite." Then, lest anyone object: are you alone the people? he adds: God has not rejected His people, that is, others besides me—there are three thousand, there are five thousand, there is a great multitude from the people who believed, as the Acts of the Apostles show (Acts 2:41, 4:4, 5:14).

After showing that the fall of the Jews is deplorable, though not entirely excusable, the Apostle now shows that it is not universal.
First, he raises a question;
second, he answers it, at God forbid;
third, he draws a conclusion, at what then.

First, therefore, he says: I say then: has God entirely cast away his people? i.e., the Jews, because he calls them unbelieving and contrary. Even the Psalmist asks: O God, why dost you cast us forever? (Ps 74:1); the Lord has scorned his altar (Lam 2:7).

Then when he says, God forbid, he answers the question and shows that God has not totally rejected the Jewish people. And this is what he says: God forbid that the Jewish people be rejected in their entirety. He proves this, first of all, with respect to himself, saying: for I also, living in the faith of Christ, am an Israelite by race: are they Israelites? So am I (2 Cor 11:22).

And because there were among the people of Israel some proselytes not descended in the flesh from the patriarchs, he says that this is not so of him, adding: of the seed of Abraham: are they descendants of Abraham? So am I (2 Cor 11:23).

Furthermore, among the Jewish people the tribes were distinguished according to the sons of Jacob, some of whom were sons of slaves and some of wives. Joseph and Benjamin were sons of Rachel, Jacob's fondest wife. Hence he shows his eminence among the Jewish people, saying: of the tribe of Benjamin: of the people of Israel, of the tribe of Benjamin (Phil 3:5). Hence, some apply to Paul what is in Genesis: Benjamin is a ravenous wolf, in the morning devouring the prey, and at evening dividing the spoil (Gen 49:27).