John 12:39

(Hom. lxviii. 2) This is a common form of speech among ourselves. I cannot love such a man, meaning by this necessity only a vehement will. The Evangelist says could not, to show that it was impossible that the Prophet should lie, not that it was impossible that they should believe.

(Tr. liii. 5) But what follows involved a deeper question: Therefore they could not believe, because that Esaias said again, He hath blinded their eyes, and hardened their hearts, that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them. That they should not believe; but if so, what sin is there in a man doing what he cannot help doing? And what is a graver point still, the cause is assigned to God; since He it is who blinded their eyes, and hardened their heart. This is not said to be the devil's doing, but God's. Yet if any ask why they could not believe, I answer, Because they would not. For as it is to the praise of the Divine will that God cannot deny Himself, so is it the fault of the human will that they could not believe.

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 hath 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 hath 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 "hath blinded their eyes, and hardened their heart"?

"They could not believe," because that Isaiah the prophet foretold it; and the prophet foretold it because God foreknew that such would be the case. But if I am asked why they could not, I reply at once, because they would not; for certainly their depraved will was foreseen by God, and foretold through the prophet by Him from whom nothing that is future can be hid.

It was not however with intent to fulfil the prophecies that the Jews slew the Lord, for in that case they would not have been impious; but it was by reason of their own malignity. For although the prophets foretold the things which were certainly to be brought to pass by their determined evil counsel, they foretold it for this cause, that the sober might leap over the pitfalls of the devil: for surely they who heard might also have taken heed. On which, account also the prediction was needful.

That it was not God Who blinded the Jews. For else He would not have required them to give account thereof, forasmuch as He surely pardons involuntary offences. But the meaning is on this wise. It is just as though Isaiah were setting before us, as having been spoken by God, the words: "If I should become a Man, and with Mine own voice expound unto you what is profitable, not even so will ye hearken unto Me, as neither did ye hearken unto the prophets; neither, when ye see signs beyond description, will ye be profited aught by seeing them." This is really what "Ye will not see" means. For He did not say: "I will harden their hearts and blind their eyes;" but He said: "Although ye hear, ye will not hear; and though ye see, ye will not see, in order that ye may not be converted and I may heal you." For if they had heard and seen in such a way as they ought, they would surely have found benefit thereby. And so the passage contains no indication of an inevitable punishment, nor does it set forth a decree of One condemning and sentencing the Jews; but it is a prediction given with a good purpose. For He knew what manner of men they were going to become, and He made a declaration concerning them. Yet the saying does not go against all [the Jews], but only against the unbelieving; for many of them have believed. In this way therefore the Seventy have rendered the passage. But it is likely that the Evangelist followed the text of the Hebrews, which differs from that of the Seventy, and therefore said: For this cause they could not believe, because: He hath blinded them; and so far as the actual wording of the prophet goes, he has not said that "God" blinded them. And it is likely that some one else did this, in order that the Jews should not convert and find healing. But even though we should accept the supposition that God blinded them, yet it must be understood in this way;----that He allowed them to suffer blinding at the hands of the devil, when they were not good as regards their character. For in this way He gives up to a reprobate mind and to passion those who are of a disposition like theirs. But whilst they were such, it was not just that they should know the depth of the mystery and its secrets, seeing that they were men that kept not even the commandments of the Law. Whereas then they received neither the Law nor the ordinances of the Gospel, closing fast the eye of their understanding; on this account they receive not the instruction that is able to illuminate them.

The same thought is expressed in the words "therefore they could not believe, because Isaiah said again of them." By all this He wants to confirm that Scripture is not false and that the prophecy of Isaiah was fulfilled exactly as he had foretold.
Lest anyone should begin to speak and wonder why Christ came at all if He knew that the Jews would not believe in Him, the Evangelist for this reason brings forward the prophets who foretold this. Christ, although He knew of their unbelief, nevertheless came so that they would have no excuse for their sin and could not say that they would have believed if He had come.
The words "they could not believe" mean the same as "they would not." For an evil and crafty man, so long as he remains such, that is, chooses evil, cannot believe.

For this reason they could not believe, in him, because evidently they were blinded, and this Isaiah said.

When the Evangelist says, therefore they could not believe, he states the prophecy which foretold the reason for their unbelief. If we examine these words of the Evangelist they seem, if taken at their surface value, difficult to understand. First, because if it is said that therefore they could not believe, because Isaiah said this, the Jews seem to be excusable. For is it a sin for a person not to do what he cannot do? And what is more serious, the fault will be cast back on God, since he blinded their eyes. This could be accepted if it were said of the devil, as in 2 Corinthians (4:4): "The god of this world has blinded the minds of the unbelievers." But here it is said of our Lord, for Isaiah (6:1) says: "I saw the Lord seated on a high and lofty throne," and follows with "Blind the heart of this people and make their ears heavy, and shut their eyes, lest they see with their eyes and hear with their ears and understand with their heart and be converted and I heal them" (Is 6:10).

To clarify this let us first explain the statement, therefore they could not believe. Here we should note that something is said to be impossible or said to be necessary in two ways: absolutely, and granted a certain presupposition. For example, it is absolutely impossible for a human being to be an ass; but granting a certain presupposition, it is impossible for me to be outside my house, presupposing, that is, that I remain within it sitting down. With this in mind, we may say that a person is excused if he does not do things that are absolutely impossible for him. But he is not excused if he does not do things that are impossible for him granting some presupposition. So, if someone has the evil intention of always stealing, and says that it is impossible for him not to sin as long as he continues with that intention, he is not excused: for this impossibility is not absolute, but based on a certain presupposition, for he can abandon his evil intention. So he says, therefore they could not believe, that is, because they had a will clouded over by their wickedness: "Can the Ethiopian change his skin or the leopard his spots? Then also you can do good who are accustomed to do evil" (Jer 13:23); "How can you do good things when you are evil?" (Mt 12:34). It is like one saying: "I can in nowise love him, because I hate him."

As to the second point, when we read that God blinds and hardens, we should not think that God puts malice into us or forces us to sin; but we should understand it as meaning that God does not infuse grace. Now he infuses grace because of his mercy, while the cause of his not infusing grace is due to us, insofar as there is something in us which opposes divine grace. As far as he is concerned: "He enlightens every man coming into this world" (1:9); "He desires all men to be saved" (1 Tim 2:4). But because we leave God, he takes his grace from us: "Because you have rejected knowledge, I reject you" (Hos 4:6), "Your destruction, O Israel, is from yourself; your help is only in me" (Hos 13:9). It is like a person who closes the shutters of his house, and someone says to him: "You cannot see because you lack the light of the sun." This would not be due to a failure of the sun, but because he shut out the light of the sun. In the same way we read here that they could not believe, because God blinded them, that is, they were the cause why they were deprived of sight as in "Their wickedness blinded them" (Wis 2:21).