Charles Ellicott 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, because.—The words refer to what has preceded, not to what follows. Then, by an addition, the reason is given more fully: “It was because of the divine will expressed in Isaiah’s prophecy.” “It was therefore, specifically, because Isaiah said again.”

The words “they could not believe” must be taken in their plain meaning as expressing impossibility. The Apostle is looking back on the national rejection of Christ and seeks a reason for it.

He remembers how our Lord Himself had explained His method of teaching by parables and based it on this prophecy of Isaiah (Matthew 13:14). The principle was that which has been repeated in His last public words (John 12:35–36): that power used is increased, and power neglected destroys itself.

Here, then, in these prophetic words was the reason they could not believe. Willful rejection had been followed by a rejection that was no longer within the power of the will. This statement of St. John's should be compared with our Lord’s words on the same subject in John 5:40 and John 6:37, and with St. Paul’s arguments in Romans 9–11.