Here is St. Thomas Aquinas’ commentary on Romans explaining this.
[quote=St. Thomas]915. Thirdly, he states what he intended.
First, with respect to the fall of particular Jews, when he says: a hardening has come upon Israel, not universally but upon a part: “Blind the heart of this people” (Is 6:10).
Secondly, he predicts the end of this blindness, saying: until the full number of the Gentiles come in to the faith, i.e., not only some Gentile nations as were then converted; but either in all or the greater part the Church would be establishes: “The earth is the Lord’s and all its fullness” (Ps 23:1).
- It should be noted that the word, until, can signify the cause of the blindness of the Jews. For God permitted them to be blinded, in order that the full number of the Gentiles come in.
It can also designate the termination, i.e., that the blindness of the Jews will last up to the time when the full number of the Gentiles will come to the faith. With this agrees his next statement, namely, and then, i.e., when the full number of the Gentiles has come in, all Israel will be saved, not some, as now, but universally all: “I will save them by the Lord their God” (Hos 1:7); "He will again have compassion upon us (Mic 7:19).
Then when he says, As it is written, he proves what he had said about the future salvation of the Jews:
first, he proves this with an authority;
secondly, with a reason [v. 28; n. 921].
First, therefore, he says: I say that all Israel will be saved, as it is written in Is (59:20), where our text says: “A redeemer will come from Zion and this will be my covenant with them that return to Jacob says the Lord.” But the Apostle uses the Septuagint and touches on three things.
First, the coming of a Savior, when he says: God will come, in human flesh to save us, from Zion, i.e., from the Jewish people who are signified by Zion, the citadel of Jerusalem, a city in Judea. Hence it says in Zech (9:9): " Rejoice greatly, o daughter of Zion! Shout aloud, O daughter of Jerusalem! Lo, our king comes to you…" and in Jn (4:22): “Salvation is from the Jews.”
Or he says that he comes from Zion, not because he was born there, but because his doctrine went from there into the whole world, inasmuch as the apostles received the Holy Spirit in the cenacle in Zion: “Out of Zion shall go forth the law” (Is 2:3).
- Secondly, he touches on salvation by Christ offered to the Jews, saying: he will banish ungodliness from Jacob. This could refer to deliverance from punishment: “He will snatch my soul from death” (Ps 115:8). Banish ungodliness from Jacob could refer to deliverance from guilt: “O that deliverance for Israel would come from Zion” (Ps 53:6).
Or both could refer to liberation from guilt, but he says he will take out, because of the few, who now are converted with great difficulty and with, so to speak, a certain violence: “As if a shepherd should get out of the lion’s mouth two legs, or the tip of the ear, so shall the children of Israel be taken out” (Amos 3:12). But he says will banish ungodliness from Jacob to show the ease with which the Jews will be converted at the end of the world: “Who is a God like thee, pardoning iniquity and passing over transgression for the remnant of his inheritance?” (Mic 7:18).
- But they are beloved by God for the sake of their forefathers as regards election, because He chose their descendants on account of their forefathers’ grace: “The Lord loved your fathers and chose their descendants after them” (Dt 10:15).
This does not means that the merits established by the fathers were the cause of the eternal election of the descendants, but that God from all eternity chose the fathers and the sons in such a way that the children would obtain salvation on account of the fathers; not as though the merits of the fathers were sufficient for the salvation of the sons, but through an outpouring of divine grace and mercy, the sons would be saved on account of the promises made to the fathers.
- Then when he says, For the gifts, he excludes an objection.
For someone might claim that even though the Jews were formerly beloved on account of their forefathers, nevertheless the hostility they exert against the gospel prevents them from being saved in the future. But the Apostle asserts that this is false, saying: The gifts and call o God are irrevocable, i.e., without repentance. As if to say: That God gives something to certain ones or call certain ones is without repentance, because God does not change His mind: "The Lord has sworn and will not change his mind” (Ps 110:4)
Then when he says, Just as you were, he gives the reason for the future salvation of the Jews after their unbelief.
First, he shows a similarity between the salvation of both people;
secondly, the cause of this similarity [v. 32; n. 932].
First, therefore, he says: So I say that Israel will be saved, although they are now enemies. For just as you Gentiles once did not believe God: “You were once without God in the world” (Eph 2:12); but now have received mercy because of their unbelief: below (15:9), “The Gentiles are to honor God for his mercy”; “I will have mercy on him who was without mercy” (Hos 2:23). And this was because of their unbelief, which was the occasion of your salvation, as was said above.
So they, i.e., the Jews, now, i.e., in the time of grace, have not believed, namely, Christ: "Why do you not believe me?” (Jn 8:46). And this is what he adds: In order that by the mercy shown to you, i.e., in Christ’s grace, by which you have obtained mercy: “You have saved us according to your mercy” (Tit 3:5). Or they have not believed so that they enter into your mercy. Or they have not believed, which turned out to be the occasion of the mercy shown to you, in order that they also at some time may receive mercy: “The Lord will have compassion on Jacob” (Is 14:1).