Romans 11:17

If some Jews have not believed, then they have been cut out of the promise.… The Gentiles, who were from a bad root, were grafted into a good tree, which is the opposite of what happens in agriculture, where it is the good branch which …

"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. […]

"And if some of the branches be broken off." And yet above thou didst say that the more part perished, and a few were saved only. How came it then that speaking of those that perished, thou hast used a "some," which is indicative of fewness? It is not, he replies, in opposition to myself, but out of a desire to court and recover those that are distressed. Observe how in the whole of the passage one finds him working at this object, the wish to solace them. And if you deny it, many contradictions will follow. But let me beg you to notice his wisdom, how while he seems to be speaking for them, and devising a solace for them, he aims a secret blow at them, and shows them to have fallen from being Abraham's kinsmen. For what he was desirous of saying was, that they had nothing in common with them. For if the root be holy, and these be not holy, then these are far away from the root. Then under the appearance of solacing the Jews, he again by his accusation smiteth them of the Gentiles. For after saying, "And if some of the branches were broken off," he proceeds. "And thou being a wild olive wert grafted in." For the less esteem the man of the Gentiles is of, the more the Jew is vexed at seeing him enjoy his goods. And to the other, the disgrace of the little esteem he was of, is nothing to the honor of the change. And consider his skilfulness. He does not say, "thou wert" planted "in," but "thou weft grafted in," by this again cutting the Jew to the heart, as showing that the Gentile man was standing in his own tree, and himself lying on the ground. […]

"The riches of the olive tree" means the teaching of righteousness.

The graft uses as soil the tree in which it is engrafted. Now all the plants sprouted forth simultaneously in consequence of the divine order. Wherefore also, though the wild olive is wild, it crowns the Olympic victors.… Now we see that the wild trees attract more nutriment because they cannot ripen. The wild trees therefore have less power of secretion than those that are cultivated. And the cause of their wildness is the absence of the power of secretion. The engrafted olive accordingly receives more nutriment from its growing in the cultivated one, and it gets accustomed, as it were, to secrete the nutriment, becoming thus assimilated to the fatness of the cultivated tree.