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 New Birth
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!   

And the Lord said unto Moses, When you go to return into Egypt, see that you do all those wonders before Pharaoh, which I have put in your hand: but I will harden his heart, that he shall not let the people go. Exo 4:21 Self Will is a gift from our Lord.     It enables us to form opinions, develop attitudes and take directions.     Self Will is, at the same time, the most wonderful and the most dangerous gift we received in our humanity.      Pride is an attitude that we willfully and purposefully choose to take on.     We can choose to exalt ourselves and we can choose to humble ourselves.      Moses did not understand the nature of our Lord because the Truth about Him was never known by any man until He manifested Himself in the flesh as our Lord Jesus Christ.      Moses did not understand that our Lord can see the end from the beginning - that He is omniscient.     So, when the Lord told Moses that Pharoah would harden His heart, Moses took it to mean that the Lord would harden Pharoah`s heart.     How else could He know what Pharoah would do?      Pharoah was full of Pride and his Pride caused him to reject Moses` call for release.      When a person operates in Pride they are resisted by God and so become self-destructive.      When a person resists God their Heart automatically hardens.      As in similar instances where God is attributed with a direct action that forces an individual in a certain direction, the reality is that the person`s Self Will developed attitude toward God has a predictable pattern of behaviour.      The original Hebrew word that was translated here as `harden` actually means `strengthen`.     So, the point was that the Lord had given Pharoah the strength of will, the Self Will, to come against Him.      In other verses where Pharoah`s heart was hardened we see that Pharoah hardened His own heart: Exo 8:15 Exo 8:32 Exo 9:34.      So, who was actually responsible for Pharoah`s hardened heart?     The Lord or Pharoah?      The answer is: both    → The Lord gave Pharoah Self Will    → Pharoah`s Self Will hardened his own heart

And he hardened Pharaoh`s heart, that he hearkened not unto them; as the Lord had said. Exo 7:13 See notes from Exo 4:21

And the Lord said unto Moses, Pharaoh`s heart is hardened, he refuses to let the people go. Exo 7:14 See notes from Exo 4:21

And the magicians of Egypt did so with their enchantments: and Pharaoh`s heart was hardened, neither did he hearken unto them; as the Lord had said. Exo 7:22

But when Pharaoh saw that there was respite, he hardened his heart, and hearkened not unto them; as the Lord had said. Exo 8:15 Here we see that Pharoah hardened his own heart.      See notes from Exo 4:21

Then the magicians said unto Pharaoh, This is the finger of the Lord: and Pharaoh`s heart was hardened, and he hearkened not unto them; as the Lord had said. Exo 8:19

And Pharaoh hardened his heart at this time also, neither would he let the people go. Exo 8:32 Again Pharoah hardens his own heart.      See notes from Exo 4:21

And Pharaoh sent, and, behold, there was not one of the cattle of the Israelites dead.     And the heart of Pharaoh was hardened, and he did not let the people go. Exo 9:7

And the Lord hardened the heart of Pharaoh, and he hearkened not unto them; as the Lord had spoken unto Moses. Exo 9:12 See notes from Exo 4:21

And when Pharaoh saw that the rain and the hail and the thunders were ceased, he sinned yet more, and hardened his heart, he and his servants. Exo 9:34 Yet again, Pharoah hardens his own heart.      See notes from Exo 4:21

And the heart of Pharaoh was hardened, neither would he let the children of Israel go; as the Lord had spoken by Moses. Exo 9:35

And the Lord said unto Moses, Go in unto Pharaoh: for I have hardened his heart, and the heart of his servants, that I might show these my signs before him: Exo 10:1 See notes from Exo 4:21

But the Lord hardened Pharaoh`s heart, so that he would not let the children of Israel go. Exo 10:20 See notes from Exo 4:21

But the Lord hardened Pharaoh`s heart, and he would not let them go. Exo 10:27 See notes from Exo 4:21

And Moses and Aaron did all these wonders before Pharaoh: and the Lord hardened Pharaoh`s heart, so that he would not let the children of Israel go out of his land. Exo 11:10 See notes from Exo 4:21

And the Lord hardened the heart of Pharaoh king of Egypt, and he pursued after the children of Israel: and the children of Israel went out with an high hand. Exo 14:8 See notes from Exo 4:21

But Sihon king of Heshbon would not let us pass by him: for the Lord your Lord hardened his spirit, and made his heart obstinate, that he might deliver him into your hand, as appears this day. Deu 2:30

Wherefore then do you harden your hearts, as the Egyptians and Pharaoh hardened their hearts?     when he had wrought wonderfully among them, did they not let the people go, and they departed? 1Sa 6:6 Here we see that the Egyptians hardened their own hearts.      See notes from Exo 4:21

Notwithstanding they would not hear, but hardened their necks, like to the neck of their fathers, that did not believe in the Lord their Lord. 2Ki 17:14

And he also rebelled against king Nebuchadnezzar, who had made him swear by the Lord: but he stiffened his neck, and hardened his heart from turning unto the Lord of Israel. 2Ch 36:13

But they and our fathers dealt proudly, and hardened their necks, and hearkened not to your commandments, Neh 9:16

And refused to obey, neither were mindful of your wonders that you did among them; but hardened their necks, and in their rebellion appointed a captain to return to their bondage: but you are a god ready to pardon, gracious and compassionate, slow to anger, and of great kindness, and forsook them not. Neh 9:17

And testified against them, that you might bring them again unto your law: yet they dealt proudly, and hearkened not unto your commandments, but sinned against your judgments, (which if a man do, he shall live in them;) and withdrew the shoulder, and hardened their neck, and would not hear. Neh 9:29

O Lord, why have you made us to err from your ways, and hardened our heart from your fear?     Return for your servants` sake, the tribes of your inheritance. Isa 63:17 Our Lord does not cause evil.     He does not harden people`s hearts.      Human rebellion against a loving, gracious Creator is so unfathomable that it appears that He must have forced us to do it.      But that is not the case.     The enemy has planted evil seed.

Yet they hearkened not unto me, nor inclined their ear, but hardened their neck: they did worse than their fathers. Jer 7:26

Thus says the Lord of hosts, the Lord of Israel; Behold, I will bring upon this city and upon all her towns all the evil that I have pronounced against it, because they have hardened their necks, that they might not hear my words. Jer 19:15

But when his heart was lifted up, and his mind hardened in pride, he was deposed from his kingly throne, and they took his glory from him: Dan 5:20

For they considered not the miracle of the loaves: for their heart was hardened. Mar 6:52 Who steals our memories of the Lord`s blessings in our past?     Satan.     In this case, the miracle of the feeding had just happened!

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 Failure to move into the Spirit Realm of understanding is due to a Hardened Heart.     A Hardened Heart refuses to see the Truth much less receive it.      As a result, the Hardened Heart is easily deceived by that which has caused it to become hardened and dulled to that which is revelation.

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 The result of rejecting Jesus Christ was that a blindness settled over the eyes of the people as they hardened their hearts against the Truth.      It appeared that the Holy Spirit did this to Israel, but they did it to themselves.      Otherwise, Jesus would not have wasted His time preaching and teaching them.      See notes from Exo 4:21

But exhort one another daily, while it is called To day; lest any of you be hardened through the deceitfulness of sin. Heb 3:13 Our job is to exhort one another in the Word Of The Lord.     Those who depart from the Word will inevitably be deceived.      The sin that hardens is unbelief.         → Unbelief in the law of the Lord.         → Unbelief in the love of the Lord.         → Unbelief in the judgment of the Lord.      Do not be offended by the Word Mat 13:21.      It is the Sword Of The Spirit Eph 6:17 and it is given to us to surgically operate on those that are weak, those who are suffering and those who are in bondage 2Ti 4:2.