Is there a chance for repentance for one whose heart has been sealed, and what are the causes, the treatment, and the struggle required for his heart to be guided to the truth and emerge from darkness?

The door of repentance is open to every living person as long as the soul is in the body and has not reached the throat. Repentance includes all sins, even disbelief, as Allah Almighty says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" The sinner must perform sincere repentance, no matter how great their sin, and know that Allah's forgiveness is greater and that Allah rejoices in their repentance. To achieve uprightness, one must observe the obligatory duties and supererogatory acts, frequently engage in remembrance (dhikr) and supplication (dua), associate with righteous people, attend gatherings of remembrance, seek religious knowledge, and distance themselves from the causes of disobedience.