The Hardened heart leads to hell and we must protect from it. Allah says:
“Has not the time come for the hearts
of those who believe to be affected by Allâh’s Reminder (this
Qur’ân) and that which has been revealed of the truth, lest they
become as those who received the Scripture
before, and the term was prolonged for them and so their
hearts were hardened? And many of them were rebellious,
disobedient (fâsiqoon)”
(57:16)