John 12:39

For this cause they could not believe, for that Isaiah had said again, 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.

They could not believe ... This accounts for the sad remark of Jesus (John 12:35). It was already too late. The people had closed their eyes, stopped their ears, and hardened their hearts. They had shut their eyes to every sign, greeted every spiritual message with some crass literalization of his words, scorned every revelation of himself as the Shepherd, the Door, the Light, the Bread of Life, etc., and had caviled at his every word. Having so hardened themselves, they inevitably suffered the penalty of God's judicial hardening, making them no longer capable of believing. For study of hardening of Israel, see my Commentary on Romans, p. 376; and for comments on the manner of God's hardening all who do not like to keep God in their hearts, see my Commentary on Romans, pp. 45f. It should ever be borne in mind that God's judicial hardening always follows, and never exists apart from the act of evil men sinfully hardening themselves.

John's quotation is from Isaiah 6:10. Matthew quoted Jesus as using exactly the same words (Matthew 13:14,15).