GOD WILL HARDEN HEARTS OF PEOPLE WHO DO NOT ACCEPT SYMBOLIC INTERPRETATION OF SCRIPTURE
Jesus criticized people who thought they had found the essence of life in the scriptures (i.e. the books of the Old/First Covenant). They had the literal understanding of scripture but no spiritual understanding. He says, in effect, that the prophecy given to Isaiah was fulfilled when the hearts of the people became hard and they lost the ability to understand with their hearts. In this prophecy God was directly responsible for the hardening of the hearts of Israel. He did that, however, because of people’s pride in thinking they knew the literal word exceedingly well. Since people thought they knew the truth, they were very careful to apply the literal truth they knew in their heavily ritualistic lives.

That is how religious behavior develops. But because they focused exclusively on what they could understand with their intellectual abilities and do with their physical bodies, God hardened their hearts so that they could not effectively know the spiritual (symbolical) truths that God was trying to communicate through symbolic language (parables and other forms of symbolism). He calls it putting a stumbling block before them. That does not seem like anything that God would do, but it is only one of several examples where He hardens hearts, so no persons should arrogantly think  that God would not harden their heart because they professes to be followers of Jesus.

Finally, Jesus said they (disciples) will be blessed when they combine what they read and hear with their natural eyes and ears with what they hear (in the still small voice of their heart) and then apply (i.e. turn, or convert) what they have learned  to their lives in a meaningful way.Sometimes God hardens the hearts of people who have already hardened them by not listening. Other times, people harden their own hearts by refusing to listen.  Whether God does the hardening or people do it to themselves, a hard heart is a bad thing to have. People with hard hearts are stuck in the letter of the law and have no concept of the Spirit of the law. Jesus said that the hearts of those who are not true disciples (i.e. they are not true learners) have become calloused to spiritual understanding because they have focused only on what they can see and hear with their natural capabilities. Another way to look at this condition is to say that they walk by sight and not by faith. It is not good to be a part of this crowd.