Could you explain how we may understand John 12:37–41?

This text is an extract from a transcript of a Q&A session given by David Gooding entitled ‘The Glorious Gospel of the Blessed God’ (1995).

The question in verse 37 is quoting Isaiah 53:1. Isaiah was astonished that, in spite of the many signs, the people wouldn't believe. John 12:39–40 says something even more serious, if that were possible: 'For this cause they could not believe, for that Isaiah 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' (cf Isaiah 6:9–10). They couldn't believe because God had hardened their hearts.

If we are to understand this remark, we must observe where it comes in the Gospel by John—there are similar remarks in the other Gospels (cf Matthew 13:13–15). It is not said at the beginning of the Gospel before Christ did his signs; it is said after Christ had done much preaching, after he had done many signs. But by that point many of the people, in spite of the preaching and in spite of the signs, still rejected Christ; and then God stepped in and hardened their hearts.

The Lord Jesus warned the people of that in the comparable passage in Matthew chapter 12. When, in spite of the signs that the Lord had done, the Pharisees said that he was doing those signs by the power of the devil (Matthew 12:24), Christ replied by warning them solemnly that if they persisted in rejecting the evidence of his signs they were sinning against the Holy Spirit. If they persisted they would not be forgiven in this age or in the age to come.

The verses in John therefore are to be understood in a similar context. They come not at the beginning of our Lord's ministry but towards its end when those who had the gospel preached to them, but knowingly had sinned against the Holy Spirit, would find that their hearts were hardened. From then on they could not believe.