We see one of the most hardened hearts in the person of Phar’-oah who fought to keep the Israelites from serving God,even after God sent ten plagues over Egypt (Exodus Chapters 7-11).
There are many that get caught up in going to a psychic or medium thinking it is harmless,but witchcraft and the like are a sin and it was the rebellion that caused Saul to be rejected from being king (1 Samuel 15:23-26). 23 For rebellion is as the sin of witchcraft, and stubbornness is as iniquity and idolatry. Because thou hast rejected the word of the Lord, he hath also rejected thee from being king.24 And Saul said unto Samuel, I have sinned: for I have transgressed the commandment of the Lord, and thy words: because I feared the people, and obeyed their voice.25 Now therefore, I pray thee, pardon my sin, and turn again with me, that I may worship the Lord.26 And Samuel said unto Saul, I will not return with thee: for thou hast rejected the word of the Lord, and the Lord hath rejected thee from being king over Israel.
It is a hardened heart that cannot understand the word of the Lord,but it can be converted and healed through repentance by the grace of God! (John 12:38-40). 38 That the saying of Esaias the prophet might be fulfilled, which he spake, Lord, who hath believed our report? and to whom hath the arm of the Lord been revealed?39 Therefore they could not believe, because that Esaias said again,40 He hath 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.