Particular believers and disbelievers were mentioned in the previous verses. Stated now is the reward and punishment of a believer and a disbeliever as a universal rule.
Commentary
Explaining the highly apocalyptic statement كُلَّمَا نَضِجَتْ جُلُودُهُم بَدَّلْنَاهُمْ in verse 56, Sayyidna Mu` adh ؓ says that as soon as their skin is burnt out it will be replaced - and this will be with such speed that the skin will be replaced a hundred times in one moment.
Sayyidna Hasan al-Basri (رح) says:
تاکل النار کلّ یوم سبعین الف مرّۃ کلما اکلتھم قیل لھم : عودوا فیعودون کما کانوا (اخرج البیھقی عین الحسن بحوالہ مظھری ج 2)
"The fire will eat them everyday seventy thousand times. When it would have eaten them up, they will be commanded: 'Return’. So, they shall return to be as they were." (al-Baihaqi, as quoted by Mazhari)
According to a narration in al-Bukhari and Muslim, the Holy Prophet ﷺ said: عن انبی ﷺ قال : ان اھون اھل النار عذابا رجل فی اخمص قدمیہ جمرتان یغلی منھما دماحہ کما یغلی الرجل بالقمقم (رواہ البخاری و مسلم ، بحوالہ الترغیب و الترھیب ج 4 ص 239)
"Lightest in punishment among the people of fire will be the man in the sole of whose feet there will be two smoldering embers the heat from which will keep his brain boiling like a cauldron boiling to the brim." (Almundhiri, al-Targhib v.4, p.239)
An explanation of 'purified wives'
According to Hakim, Sayyidna Abu Said al-Khudri ؓ narrated that the Holy Prophet ﷺ said: The women of Jannah will be pure, that is, pure from menstruation, defecation, urination and nasal mucus.
Mujahid adds to the above by saying: They shall also be 'pure and free from procreation and seminal impurity.' (Mazhari)
Let us now look at the last two words: ظِلًّا ظَلِيلًا in verse 57. Here, by mentioning ظلیل after ظل it has been mentioned that the shade in Jannah will be dense and ever-present. This is like saying: شَمسُ شَامِسُ or لیللُ لِیٔیلُ which indicate that the blessings of Paradise will last forever.
Sayyidna Abu Hurairah narrates that the Holy Prophet ﷺ said: اِن فی الجنۃ الشجرۃ یسیر الراکب فی ظلھا مأۃ عام ما یقطعھا، اقروا إن شٔتم و ظل ممدود
"There is a tree in Paradise. If a rider were to ride under its shade for a hundred years, he will still not cross it. Read, if you wish, وَظِلٍّ مَّمْدُودٍ (A shade, ever-extended, ever-stretched)."
Explaining ظِلًّا ظَلِيلًا ، Rabi' ibn Anas said:
ھو ظل العرش الذی لا یزول
"That is the shade of the Throne (al-'arsh) which will never decline."