'Hardness' in the Bible

After looking around at them in anger, grieved by the hardness of their hearts, he said to the man, "Stretch out your hand." He stretched it out, and his hand was restored.

Then he appeared to the eleven themselves, while they were eating, and he rebuked them for their unbelief and hardness of heart, because they did not believe those who had seen him resurrected.

They are darkened in their understanding, being alienated from the life of God because of the ignorance that is in them due to the hardness of their hearts.