5 But the words of the Gospel also, that follow, are still more pressing, and start a question of more profound import: for He goes on to say, “Therefore they could not believe, because that Isaiah said again, He has blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them.” For it is said to us: If they could not believe, what sin is it in man not to do what he cannot do and if they sinned in not believing, then they had the power to believe, and did not use it.
If, then, they had the power, how says the Gospel, “Therefore they could not believe, because that Isaiah said again, He has blinded their eyes, and hardened their heart;” so that (which is of grave import) to God Himself is referred the cause of their not believing, inasmuch as it is He who “has blinded their eyes, and hardened their heart”? For what is thus testified to in the prophetical Scriptures, is at least not spoken of the devil, but of God. For were we to suppose it said of the devil, that he “has blinded their eyes, and hardened their heart;” we have to undertake the task of being able to show what blame was theirs in not believing, of whom it is said, “they could not believe.”
And then, what reply shall we give touching another testimony of this very prophet, which the Apostle Paul has adopted, when he says: “Israel has not obtained that which he seeks for; but the election has obtained it, and the rest were blinded, according as it is written, God has given them the spirit of remorse, eyes that they should not see, and ears that they should not hear, unto this day”?
Source: Tractates on the Gospel of John (New Advent)