Expositor's Bible Commentary Commentary John 12:40

"He hath blinded their eyes, and he hardened their heart; Lest they should see with their eyes, and perceive with their heart, And should turn, And I should heal them." — John 12:40 (ASV)

Not only did prophecy describe unbelief, it also explained it. Why should the hearers of Jesus not believe in him when the signs so unmistakably accredited his claims? John quotes from Isa 6 (the story of Isaiah’s commission) to show that unbelief is the result of the rejection of light, which act, by the sovereign law of God, gradually makes belief impossible. God warned the prophet in advance that his mission would not be successful, for the hearts of those who heard him would be hardened. John interprets this prophecy by its effect rather than by its intention. It was not God’s desire to alienate his people; but without the offer of faith and repentance, they would never turn to him anyway. The cumulative effect of unbelief is a hardened attitude that becomes more impenetrable as time progresses.