6 Such, as you have just heard, brethren, is the question that comes before us, and you can perceive how profound it is; but we shall give what answer we can. “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.
But the prophet, do you say, assigns another cause than that of their will. What cause does the prophet assign? That “God has given them the spirit of remorse, eyes that they should not see, and ears that they should not hear; and has blinded their eyes, and hardened their heart.” This also, I reply, their will deserved. For God thus blinds and hardens, simply by letting alone and withdrawing His aid: and God can do this by a judgment that is hidden, although not by one that is unrighteous.
This is a doctrine which the piety of the God-fearing ought to preserve unshaken and inviolable in all its integrity: even as the apostle, when treating of the same intricate question, says, “What shall we say then? Is there unrighteousness with God? God forbid.” If, then, we must be far from thinking that there is unrighteousness with God, this only can it be, that, when He gives His aid, He acts mercifully; and, when He withholds it, He acts righteously: for in all He does, He acts not rashly, but in accordance with judgment.
And still further, if the judgments of the saints are righteous, how much more those of the sanctifying and justifying God? They are therefore righteous, although hidden. Accordingly, when questions of this sort come before us, why one is dealt with in such a way, and another in such another way; why this one is blinded by being forsaken of God, and that one is enlightened by the divine aid vouchsafed to him: let us not take upon ourselves to pass judgment on the judgment of so mighty a judge, but tremblingly exclaim with the apostle, “O the depth of the riches both of the wisdom and knowledge of God! How unsearchable are His judgments, and His ways past finding out!” As it is also said in the psalm, “Your judgments are as a great deep.”
Source: Tractates on the Gospel of John (New Advent)