Rom 11:23

“"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. You see too how he raises up even him in his despondency, and humbles the other in his confidence; and do not thou be faint at hearing of severity, nor thou be confident at hearing of goodness. The reason why He cut thee off in severity was, that thou mightest long to come back. The reason why He showed goodness to thee was, that thou mightest continue in (he does not say the faith, but) His goodness, that is, if thou do things worthy of God's love toward man. For there is need of something more than faith.”

“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. Most wisely Paul frightens the Gentile by what happened to the Jews, while to the Jew, by the example of what happened to the Gentiles, he inspires the boldness to be grafted in by faith and encourages him with hope in the power of God. God is able, he says, to graft them in; He does all things that surpass our hopes.”

“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). Then he proves what he has said: first, from God's power, saying: for God is able to graft them in again; therefore, their salvation is not to be despaired of: behold, the Lord's hand is not shortened, that it cannot save (Isa 59:1).”