For they considered not the miracle of the loaves: for their heart was hardened. - Mar 6:52
And when Jesus knew it, he says unto them, Why reason you, because you have no bread? perceive you not yet, neither understand? have you your heart yet hardened? - Mar 8:17
He has blinded their eyes, and hardened their heart; that they should not see with their eyes, nor understand with their heart, and be converted, and I should heal them. - Joh 12:40
To harden something is to change it from a softer, malleable state to a state that is firmer and resistant to Change.
Education and Experience tend to harden us just as heating and cooling steel causes it to harden.
Receiving a certain amount of Education has become a legal requirement in civilized countries. Experience is unavoidable.
Education may be Good or Evil. It depends on what is being taught.
Experiences may be Good or Evil. It depends on the nature of the Experience.
There are three more factors that can harden us which are 'optional' in the sense that they are not required by Society and must be willingly chosen.
1. Relationship with God through the NewBirth
2. Hearing the Word of God
3. Praying in the Spirit.
In essence, a Heart may be hardened against God or for God.
Choose to harden your heart for God so that He may Manifest His Glory in and through you!