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ʾinna lladhīna kafarū bi-ʾāyātinā sawfa nuṣlīhim nāran kullamā naḍijat julūduhum baddalnāhum julūdan ghayrahā li-yadhūqū l-ʿadhāba ʾinna llāha kāna ʿazīzan ḥakīman
Indeed We shall soon make those who deny Our signs enter a Fire: as often as their skins become scorched, We shall replace them with other skins, so that they may taste the punishment. Indeed Allah is all-mighty, all-wise.
— Ali Quli Qarai
Surah An-Nisa', Verse 56
56. "Verily those who disbelieved in Our Signs, soon We will cast them into Fire so oft as their skins shall be burnt We will change them for other skins, that they may taste the torment. Surely Allah is the Mighty, the Wise.".
Since the infidels were constantly obstinate, they should be retributed constantly with their new skins. Once, Ibn-i-Abil-'Uja', who was one of the opponents of the religion, asked Imam Sadiq (as) what the fault of the new skin was. Imam (as) said: "The new skin is from the remaining of the burnt of the very first skin. It is like a sun-dried brick which breaks into pieces and will be made of it mud and a new sun-dried brick again."
Explanations
1. The punishment of the disbelievers is permanent.
2. The punishment, because of its continuation, will never become ordinary for the disbelievers.
3. The new skin is for tasting a severe chastisement; (since, in burning, the utmost pain is felt in the skin, and when the fire reaches the bone its pain seems less than before).
4. The Resurrection will be bodily, because the punishment will be upon the skin and the change that it will have. .