Question: Are Paradise and Hell not eternal?
There are many Qur’anic verses confirming that Paradise and Hell are eternal, e.g., Al-Baqara 25, Al-i ‘Imran 116, Al-Maidah 85, Al-An’am128, At-Tawbah 68, Hud 107.

“Hum feeha khalidoon [which means they will remain therein eternally]” is stated in the Qur’anic verse for both Hell and Paradise. (Al-Baqara 81, 82)

The Qur’an al-karim states that disbelievers will stay in Hell eternally and that Believers will stay in Paradise eternally. In this case, is it not that the attribute endlessness has been used for beings besides Allahu ta’ala as well?
As their existence and keeping on existing is not from themselves, so their being eternal is not from themselves. It is Allahu ta’ala who renders them eternal. If Allahu ta’ala says, “Be!” they come into existence. If He says, “Cease to exist!” they cease to exist. Hadrat Imam-i Rabbani says:

It is one of the conditions of faith (iman) to believe that creatures will be annihilated as well as to believe that they were created from nothing. The statement “The Arsh, the Kursi, the Lawh, the Pen, Paradise, Hell, and the Ruh will not be annihilated, and they will remain eternally” does not mean that these cannot be annihilated. Allahu ta’ala will annihilate whichever He wishes of the things that He created and He will never annihilate some others that He wishes, for uses and reasons which He, alone, knows; these will eternally exist. Allahu ta’ala does what He wishes and commands what He wishes. As understood from what has been written so far, the ‘alam, that is, everything, exists through Allah’s Will and Power. Everything needs Allahu ta’ala to exist and to remain in existence because to be eternal means to go on existing every moment. It does not mean to become something else. It is by Allah’s Will and Decree both to exist and to go on existing. (Vol. 3, Letter 57)

Both Paradise and Hell will be filled
Some people claim, “When there are empty and unoccupied places in Paradise, new people will be created to fill them,” while some others claim, “All the people in Hell will get out. Thus, Paradise will be filled, and there will be no one in Hell.” Which one is true?
Neither of them is true. Allahu ta’ala will fill both Paradise and Hell. Believers will stay in Paradise eternally, and their numbers will never decrease. Disbelievers, too, will stay in Hell eternally, and their numbers will never decrease. Only sinful Believers, after paying the penalty in Hell for what they did, will get out of Hell. Then they will be admitted to Paradise and will stay there eternally.

The following is a hadith-i sharif confirming that both Paradise and Hell, which will exist eternally, will be filled:
(Paradise presented some evidence and said, “The weak and the poor will enter me.” And Hell said, “The oppressive and the arrogant will enter me.” Then Allahu ta’ala stated, “O Hell, you are My [means of] torment by which I punish [disbelievers] whomever I wish. O Paradise, you are My mercy by which I show mercy to [Believers] whomever I wish. I will fill both of you.”) [Muslim]

The mercy of Allah
Some people affirm, “Allah, who has infinite mercy, does not burn His slaves, whom He Himself created, in Hell, even if they are disbelievers.” Is it true? Will disbelievers not stay in Hell forever?
Disbelievers will stay in Hell forever. What such people affirm is very wrong. Of course Allahu ta’ala possesses infinite mercy, but on the other hand He Himself explained to whom and how He will show mercy. Two Qur’anic verses say (what means):
(On that day, My mercy will be granted only to those who, fearing Me, abstained from being disbelievers and from sinning, gave zakat, and believed in the Qur’an al-karim and the Messenger, the illiterate Prophet.) [Al-A’raf 156, 157]

(Allah will never forgive disbelievers and the unjust. He will guide them to the path of Hell, wherein they will dwell eternally. And this is easy for Allah.) [An-Nisa’ 168, 169]

Allahu ta’ala is merciful, but at the same time He is vindictive and has no need for anything. He does not fear anybody in doing what He wishes. (Riyad-un-nasihin)

Is Hell not eternal?
Some people say, “It is said in the 23rd verse of Naba’ Sura that people will remain in Hell not forever, but for centuries. Hence, disbelievers will not remain in Hell eternally, either.” Can it be true?
No, it is not true. The purport of the verse mentioned is as follows:
(They will remain therein for ahqaab [for huqubs, for ages].) [An-Naba’ 23]

The word ahqaab is the plural of the word huqubHuqub means an age that contains many years. Hadrat Imam-i Qurtubi writes the following in his exegesis:

What is meant by “They will remain therein for ages” is that they will stay in Hell for as long as ages last, and these ages will go on incessantly. When one age comes to an end, another age will follow it. It means that days will come one after another endlessly. If it had been said “five ahqaab or ten ahqaab” or if something similar had been used, then it would have referred to a specific length of time. The reason why the word ahqaab was used is that huqub referred to the longest period of time according to them [people in those times]. Thus, they were addressed in a manner that fit with their mindsets and they were familiar with. This phrase was used to refer to eternity. That is, they will remain therein eternally, without end. The word ahqaab causes consternation in hearts to a much more degree and is more indicative of endlessness. The word ahqaab was used since these meanings were similar. (Jami’u li-Ahkam)

The exegeses BaydawiJalalayn, and Madarik say, “They will remain therein for endless ages.” This point is expressed more clearly in the following Qur’anic verses (which mean):

(Those who are completely encompassed by their evil 
[their sins and disbelief] are the people of Hell. They will abide therein eternally.) [Al-Baqara 81]

(Surely the torment of Hell will be eternal, without end for disbelievers.) [Az-Zukhruf 74]