John 12:40

He hath blinded their eyes - The expression in Isaiah is, “Go, make the heart of this people fat, and shut their eyes.” That is, go and proclaim truth to them truth that will result in blinding their eyes. Go and proclaim the law and the will of God, and the effect will be, owing to the hardness of their heart, that their eyes will be blinded and their hearts hardened. As God knew that this would be the result - as it was to be the effect of the message, his commanding Isaiah to go and proclaim it was the same in effect, or in the result, as if he had commanded him to blind their eyes and harden their hearts. It is this effect or result to which the evangelist refers in this place. He states that God did it, that is, he did it in the manner mentioned in Isaiah, for we are limited to that in our interpretation of the passage. In that case it is clear that the mode specified is not a direct agency on the part of God in blinding the mind - which we cannot reconcile with any just notions of the divine character - but “in suffering the truth to produce a regular effect on sinful minds, without putting forth any positive supernatural influence to prevent it.” The effect of truth on such minds is to irritate, to enrage, and to harden, unless counteracted by the grace of God. See Romans 7:8, Romans 7:11; 2 Corinthians 2:15. And as God knew this, and, knowing it, still sent the message, and suffered it to produce the regular effect, the Evangelist says “he hath blinded their minds,” thus retaining the substance of the passage in Isaiah without quoting the precise language; but in proclaiming the truth there was nothing wrong on the part of God or of Isaiah, nor is there any indication that God was unwilling that they should believe and be saved.

That they should not see ... - This does not mean that it was the design of God that they should not be converted, but that it was the effect of their rejecting the message. See the notes at Matthew 13:14.