Commentary on John
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.
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Commentary on John
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).
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