For if thou wert cut out of the olive tree which is wild by nature, and wert graffed contrary to nature into a good olive tree: how much more shall these, which be the natural branches, be graffed into their own olive tree? The olive tree, which is wild by nature - Which is κατα φυσιν, naturally, wild and barren; for that the wild olive bore no fruit is sufficiently evident from the testimony of the authors who have written on the subject; hence the proverb, Ακαρποτερος αγριππου· more unfruitful than the wild olive. Λακωνες γαρ αγριαν ελαιαν αγιππον καλουσι· for the Lacedemonians term the wild olive αγριππον. See Suidas. And hence Hesychius interprets αγριελαιος, the wild olive, (the word used here by St. Paul), by ακαρπος, unfruitful: and the reason given in Diogen. Proverb. Cent. ii. n. 63, is φυτον γαρ εστιν ὁ αγριππος ακαρπον· for the wild olive is an unfruitful tree. On this account the apostle very properly says: Thou wert cut, εκ της κατα φυσιν αγριελαιου, out of that olive which is uncultivated, because it is barren: the κατα φυσιν does not refer here to its being naturally barren; but to its being commonly or customarily permitted to remain so. And that this is the import of the phrase here is evident from the next clause of the verse.
And wert grafted contrary to nature - Παρα φυσιν, contrary to all custom; for a scion taken from a barren or useless tree is scarcely ever known to be grafted into a good stock; but here the Gentiles, a fruitless and sinful race, are grafted on the ancient patriarchal stock. Now, if it was possible to effect such a change in the state and disposition of the Gentiles, who were αθεοι εν τῳ κοσμῳ, Ephesians 2:12, without God, Atheists, in the world; how much more possible is it, speaking after the manner of men, to bring about a similar change in the Jews, who acknowledge the one, only, and true God, and receive the law and the prophets as a revelation from him. This seems to be the drift of the apostle's argument.