Romans 11:23

"But if the firstfruits are holy, so is the lump; and if the root is holy, so are the branches. But if some of the branches were broken off, and you, though you were a wild olive, were grafted in among them and became a sharer in the root and the fatness of the olive tree, do not boast against the branches. But if you do boast, it is not you who bear the root, but the root you. You say then: The branches were broken off so that I might be grafted in. Well: they were broken off because of unbelief. But you stand by faith: do not be proud in your thinking, but fear. For if God did not spare the natural branches, neither will he spare you. Behold, then, the goodness and the severity of God: toward those indeed who fell, severity; but toward you, goodness, if you continue in goodness. Otherwise you too will be cut off, and they, if they do not continue in unbelief, will be grafted in. For God is able to graft them in again. For if you were cut out of the olive tree which is wild by nature, and contrary to nature were grafted into the good olive tree, how much more shall these, who are according to nature, be grafted into their own olive tree?" I do not know how those who come from the school of Valentinus and Basilides, failing to hear these things thus spoken by Paul, suppose that there is a nature of souls which is always saved and never perishes, and another which always perishes and is never saved: when Paul plainly says that the branches of the good olive tree were broken off because of their own unbelief, and stood in need of the vengeance of divine severity; and again that the branches of the wild olive — which among them is set down as a nature that is lost — were grafted into the root and fatness of the olive tree. But to those people, indeed, an easy answer may be given by these words. […]

Paul shows what an easy matter this would be for God to accomplish.

"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 …

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 …