John Chrysostom

"Now if the fall of them be the riches of the world, and the diminishing of them the riches of the Gentiles, how much more their fulness?" Here he is speaking to gratify them. For even if these had fallen a thousand times, the Gentiles would not have been saved unless they had shown faith. As the Jews likewise would not have perished unless they had been unbelieving and disputatious. But as I said, he is solacing them now they are laid low, giving them so much the more ground to be confident of their salvation if they altered. …

Homily on Romans 19 ↗

Theophylact of Ohrid

If, he says, the Jews, having stumbled, prepared salvation for the Gentiles, and the Gentiles were accepted when the Jews were rejected, and the fall of the Jews became the riches of the Gentiles, then how much more their "fullness," that is, when they, having turned back, shall all be saved. He says this to please the Jews and to console them. For although the Jews fell, the Gentiles would not have been saved if they had not had faith.

Commentary on Romans ↗

Thomas Aquinas

Then when he says, now if the offense of them, he answers the question as interpreted in the second way and shows that the fall of the Jews is reparable. He does this in three ways: first, from its usefulness; second, from the Apostle's intention, at for I say to you; third, from the condition of that people, at for if the firstfruit be holy. …

Commentary on Romans ↗