John Calvin Commentary John 12:39

"For this cause they could not believe, for that Isaiah said again," — John 12:39 (ASV)

Therefore they could not believe. This is somewhat harsher; because, if the words are taken in their natural meaning, the way was shut up against the Jews, and the power of believing was taken from them, because the prophet's prediction adjudged them to blindness before they determined what choice they should make.

I reply, there is no absurdity in this, if nothing could happen differently from what God had foreseen. However, it should be observed that God's mere foreknowledge is not in itself the cause of events; in this passage, we should consider not so much God's foreknowledge as His justice and vengeance. For God does not declare what He observes from heaven that men will do, but what He Himself will do—namely, that He will strike wicked men with giddiness and stupidity, and thus will take vengeance on their obstinate wickedness. In this passage, the prophet points out the more immediate and secondary cause why God intends His word, which is in its own nature beneficial and life-giving, to be destructive and deadly to the Jews: it is because they deserved it by their obstinate wickedness.

It was impossible for them to escape this punishment, because God had once decreed to give them over to a reprobate mind and to change the light of His word, so as to make it darkness to them. For this latter prediction differs from the former in this respect: in the former passage, the prophet testifies that no one believes except those whom God, by His free grace, enlightens for His own good pleasure, the reason for which does not appear. For since all are equally ruined, God, of His mere good pleasure, distinguishes from others those whom He sees fit to distinguish. But, in the latter passage, the prophet speaks of the hardness by which God has punished the wickedness of an ungrateful people. Those who do not pay attention to these distinctions mistake and confuse passages of Scripture that are quite different from each other.