Romans 11:21

"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. […]

Judgment will be in accordance with grace, and the Judge will make examination of how you have used the graces bestowed upon you.

"For if God spared not the natural branches," and then he does not say, neither will He spare thee," but "take heed, lest He also spare not thee." So paring away the distasteful from his statement, representing the believer as in the struggle, he at once draws the others to him, and humbles these also.

The reason for this admonition is given when he says: for if God has not spared the natural branches, i.e., the Jews, who descended by natural origin from the patriarchs, but allowed them to be broken off, fear lest perhaps also he does not spare …

The Jews were sons of the patriarchs by nature, yet were broken off by unbelief. All the more should you fear, you who are grafted in, whether God will spare you in the case of your fall.