John Chrysostom

"And they also, if they abide not in unbelief, shall be grafted in." For it was not God that cut them off, but they have broken themselves off and fallen, and he did well to say have broken themselves off. For He hath never yet so cast them off, though they have sinned so much and so often. You see what a great thing a man's free choice is, how great the efficacy of the mind is. For none of these things is immutable, neither thy good nor his evil. …

Homily on Romans 19 ↗

Theophylact of Ohrid

So too the Jews would have been grafted in, if they had not continued to remain in unbelief; for God did not first cut them off, but they themselves fell away of their own accord and were cut off by their unbelief. In saying that "God did not spare," he means that God does not show mercy to the Jews, but considers them unworthy of communion and fellowship with the holy forefathers. …

Commentary on Romans ↗

Thomas Aquinas

Second, he shows the same with respect to the Jews. First, he states the fact: and they also, namely, the Jews, if they do not abide still in unbelief, shall be grafted in, i.e., restored to their former status: you have prostituted yourself to many lovers. Nevertheless, return to me, says the Lord (Jer 3:1). …

Commentary on Romans ↗