Is there repentance for one whose heart has hardened or become veiled, or for one whose heart Allah has sealed, and what are the causes and signs leading to that, and what are the types of hearts and the characteristics of each?

The answer explains that Allah accepts the repentance of all His servants, even disbelievers and hypocrites, citing noble verses such as the Almighty's saying: (Except for those who repent and correct themselves and hold fast to Allah and make their religion sincerely for Allah – then those will be with the believers) [An-Nisa: 146], and (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.") [Az-Zumar: 53].

As for the causes of the sealing of the heart, they are sins and disobedience, evidenced by the Prophet's (peace be upon him) hadith: "When a servant commits a sin, a black spot is marked on his heart. If he refrains, seeks forgiveness, and repents, his heart becomes polished. If he repeats it, it increases until it covers his heart, and that is the 'Ran' that Allah mentioned: (No! Rather, what they used to earn has covered their hearts.)"

The signs of a sealed heart are a lack of response to Allah's commands. The answer refers to the categories of hearts as mentioned by Ibn Al-Qayyim: the sound heart, the dead heart, and the sick heart. The treatment for hearts is through reciting the Holy Quran, for it is a healing for what is in the breasts from doubt and removes polytheism and the ailments of suspicions and desires.