2 But that the root was in the Patriarchs, how shall we show? Let us question Paul. The Gentiles now believing in Christ, and desiring as it were to boast over the Jews who crucified Christ; although also from that same people there came another wall, meeting in the corner, that is, in Christ Himself, the wall of uncircumcision, that is, of the Gentiles, coming from a different quarter: when, I say, the nations were lifting up themselves, he does thus depress them. “For if you,” he says, “being cut out of the natural wild olive, hast been graffed in among them, do not boast against the branches: for if you boast, thou dost not bear the root, but the root you.” Therefore he speaks of certain branches broken off from the root of the Patriarchs because of unbelief, and the wild olive therein graffed in, that it might be partaker of the fatness of the olive, that is, the Church coming out of the Gentiles.
And who does graff the wild olive on the olive? The olive is wont to be graffed on the wild olive; the wild olive on the olive we never saw. For whosoever may have done so will find no berries but those of the wild olive. For that which is graffed in, the same grows, and of that kind the fruit is found. There is not found the fruit of the root but of the graft. The Apostle showing that God did this thing by His Omnipotence, namely, that the wild olive should be graffed into the root of the olive, and should not bear wild berries, but olive— ascribing it to the Omnipotence of God, the Apostle says this, “If you have been cut out of the natural wild olive and against nature hast been graffed into a good olive, do not boast,” he says, “against the branches.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)