The Bible often describes people, who are obstinate towards God, as being hardhearted and the main cause of unbelief in God. When you take the time to study the Greek words behind "hardening of the heart," there are a range of meanings:

Making strong or rigid.

Stubborn, stiff-necked and obstinate.

More than a mere stiffening of one's soul and character - an implication that there is the outgrowth of a new feature, which obscures while it hardens, of an outer coating of mental obduracy or permanent bluntness.

1. Carefully observe the warning given by the writer of Hebrews. Is he writing to Believers? What do you learn about "you may be hardened by the deceitfulness of sin?"

Take care, brothers, lest there be in any of you an evil, unbelieving heart, leading you to fall away from the living God. But exhort one another every day, as long as it is called "today," that none of you may be hardened by the deceitfulness of sin. (Heb 3:12-13, ESV)

But the serpent said to the woman, "You will not surely die. For God knows that when you eat of it your eyes will be opened, and you will be like God, knowing good and evil." So when the woman saw that the tree was good for food, and that it was a delight to the eyes, and that the tree was to be desired to make one wise, she took of its fruit and ate, and she also gave some to her husband who was with her, and he ate. (Gen 3:4-6, ESV)

Blessed is the one who fears the Lord always,
but whoever hardens his heart will fall into calamity. (Prov 28:14, ESV)

2. Examine what senses are involved here. What do you learn about hardheartedness?

And Jesus, aware of this, said to them, "Why are you discussing the fact that you have no bread? Do you not yet perceive or understand? Are your hearts hardened? Having eyes do you not see, and having ears do you not hear? And do you not remember? When I broke the five loaves for the five thousand, how many baskets full of broken pieces did you take up?" They said to him, "Twelve." "And the seven for the four thousand, how many baskets full of broken pieces did you take up?" And they said to him, "Seven." And he said to them, "Do you not yet understand?" (Mark 8:17-21, ESV)

Now when he rose early on the first day of the week, he appeared first to Mary Magdalene, from whom he had cast out seven demons. She went and told those who had been with him, as they mourned and wept. But when they heard that he was alive and had been seen by her, they would not believe it.
After these things he appeared in another form to two of them, as they were walking into the country. And they went back and told the rest, but they did not believe them.
Afterward he appeared to the eleven themselves as they were reclining at table, and he rebuked them for their unbelief and hardness of heart, because they had not believed those who saw him after he had risen. (Mark 16:9-14, ESV)