John 12:40-41

40. He hath blinded their eyes, and hardened their heart. The passage is taken from Isaiah 6:9, where the Lord forewarns the prophet, that the labor which he spends in instructing will lead to no other result than to make the people worse. First then he says, Go, and tell this people, Hearing, hear and do not hear; as if he had said, “I send thee to speak to the deaf.” He afterwards adds, Harden the heart of this people, &c. By these words he means, that he intends to make his word a punishment to the reprobate, that it may render them more thoroughly blind, and that their blindness may be plunged in deeper darkness. It is indeed a dreadful judgment of God, when He overwhelms men by the light of doctrine, in such a manner as to deprive them of all understanding; and when, even by means of that which is their only light, he brings darkness upon them.

But it ought to be observed, that it is accidental to the word of God, that it blinds men; for nothing can be more inconsistent than that there should be no difference between truth and falsehood, that the bread of life should become a deadly poison, and that medicine should aggravate a disease. But this must be ascribed to the wickedness of men, which turns life into death. It ought also to be observed, that sometimes the Lord, by himself, blinds the minds of men, by depriving them of judgment and understanding; sometimes by Satan and false prophets, when he maddens them by their impostures; sometimes. too by his ministers, when the doctrine of salvation is injurious and deadly to them. But provided that prophets labor faithfully in the work of instruction, and commit to the Lord the result of their labor, though they may not succeed to their wish, they ought not to give way or despond. Let them rather be satisfied with knowing that God approves of their labor, though it be useless to men’ and that even the savor of doctrine, which wicked men render deadly to themselves elves, is good and pleasant to God, as Paul testifies, (2 Corinthians 2:15.)