John 12:39

Therefore they could not believe, because that Esaias said again, &c. The words "therefore" and "because" signify not the cause of their unbelief, as Calvin supposes, but marking the necessary consequence. It could not but be so, because it had been foretold, and Scripture cannot lie. But God foretold it, because He foresaw that through their freedom of will, their obstinacy and malice, they would not believe in Christ. God therefore saw that they would not believe, because they, of their own free will, would not do so. But they did not refuse to believe, because God foresaw that they would not believe. For their unbelief was prior to God's foreseeing. God foresees the future, because it will surely come to pass. For God cannot foresee anything, unless it is presupposed that it will really take place. For the object which is seen is prior to the act of seeing it. For nothing can be seen but that which either now is, or hereafter will be. So S. Chrysostom, Jansenius, Maldonatus, and others.

But S. Augustine, and after him Toletus, explain it thus: the Jews could not believe in Christ, because they were hardened and blinded, as Isaiah foretold. But then the words "could not" do not signify absolute necessity, but either a moral, that is a great, difficulty, or else a conditional difficulty. That is to say, the Jews could not believe in Christ, supposing they continued to hold fast to their sins, darkness, and ignorance; and therefore blinded and hardened themselves by their own wickedness. For otherwise, though they were blinded and hardened, yet as having free will, and sufficient grace to enable them, they could (speaking abstractedly) give up their hardness of heart and turn to God.

He hath blinded their eyes, and hardened their hearts. Christ quotes Isaiah 6:9-10. Having fully explained this passage before, I will here briefly repeat what I there said. Observe then (1) that properly speaking the intellect is said to be blinded, but the affections and will to be hardened; (2) that the direct and proper cause of a man's blindness and hardening, is his own free will and wickedness. See Wisdom 2: 21. The Arabic and Syriac versions understand it in this way, "their eyes are blinded, and their heart is hardened." But yet God is said indirectly and in a less strict sense (improprie) to harden a man, because He gradually withdraws from Him the light of truth and grace, and allows opportunities of error and sin to be presented to him by the world, the flesh, and the devil, in punishment for his former sins.

Moreover, in Isaiah we read "blind thou the heart of this people," these being the words of God to Isaiah. But it comes to the same meaning. For "blind thou," is the same as "foretell that a man will, indirectly, be blinded by Me." "He blinded" is then the same as " He will blind." The past is put for the future, to signify the certainty of the thing, that it will as surely come to pass as though it had already happened; that the Jews will be as surely blinded, as though they had been blinded already.