Allah, the Exalted, says,

وَأَمَّا الَّذِينَ سُعِدُواْ

(And those who are blessed.) These are the followers of the Messengers.

فَفِى الْجَنَّةِ

(they will be in Paradise,) This means that their final abode will be Paradise.

خَـلِدِينَ فِيهَآ

(abiding therein for all the time) This means that they will remain there forever.

مَا دَامَتِ السَّمَـوَتُ وَالاٌّرْضُ إِلاَّ مَا شَآءَ رَبُّكَ

(that the heavens and the earth endure, except as your Lord wills:) The meaning of the exception that is made here is that the condition of eternal pleasure that they will experience therein is something that is not mandatory by itself. Rather, it is something that is dependent upon the will of Allah. Unto Him belongs the favor of immortality upon them. For this reason they are inspired to glorify and praise Him, just as they are inspired to breathe. Ad-Dahhak and Al-Hasan Al-Basri both said, "It is about the right of the disobedient people of Tawhid who were in the Fire and then brought out of it." Then Allah finished this statement by saying,

عَطَآءً غَيْرَ مَجْذُوذٍ

(a gift without an end.) This means that it will never be cut off. This has been mentioned by Mujahid, Ibn `Abbas, Abu Al-`Aliyah and others. This has been mentioned so that the suspicious person will not doubt after the mention of the will of Allah. Someone may think that the mention of Allah's will here means that the pleasure of Paradise may end or change. To the contrary, it has been decreed that this pleasure will truly be forever and will never end. Likewise, Allah has clarified here that the eternal torment of the people of the Fire in Hell also is due to His will. He explains that He punishes them due to His justness and wisdom. This is why He says,

إِنَّ رَبَّكَ فَعَّالٌ لِّمَا يُرِيدُ

(Verily, your Lord is the doer of whatsoever He intends.) Similarly, Allah says,

لاَ يُسْأَلُ عَمَّا يَفْعَلُ وَهُمْ يُسْـَلُونَ

(He cannot be questioned as to what He does, while they will be questioned.)21:23 Here, Allah soothes the hearts and affirms the intent, by His saying,

عَطَآءً غَيْرَ مَجْذُوذٍ

(a gift without an end.) It has been recorded in the Two Sahihs that the Messenger of Allah ﷺ said,

«يُؤتَى بِالْمَوْتِ فِي صُورَةِ كَبْشٍ أَمْلَحَ فَيُذْبَحُ بَيْنَ الْجَنَّةِ وَالنَّارِ،ثُمَّ يُقَالُ: يَا أَهْلَ الْجَنَّةِ خُلُودٌ فَلَا مَوْتَ، وَيَا أَهْلَ النَّارِ خُلُودٌ فَلَا مَوْت»

(Death will be brought in the form of a handsome ram (on the Day of Judgement) and it will be slaughtered between Paradise and the Hellfire. Then, it will be said, "O people of Paradise! Eternity and no death! O people of Hellfire! Eternity and no death!") In the Sahih it is recorded that the Messenger of Allah ﷺ said,

«فَيُقَالُ: يَا أَهْلَ الْجَنَّةِ إِنَّ لَكُمْ أَنْ تَعِيشُوا فَلَا تَمُوتُوا أَبَدًا، وَإِنَّ لَكُمْ أَنْ تَشِبُّوا فَلَا تَهْرَمُوا أَبَدًا، وَإِنَّ لَكُمْ أَنْ تَصِحُّوا فَلَا تَسْقَمُوا أَبَدًا، وَإِنَّ لَكُمْ أَنْ تَنْعَمُوا فَلَا تَبْأَسُوا أَبَدًا»

(It will be said, `O people of Paradise, verily you will live and you will never die. You will remain young and you will never grow old. You will remain healthy and you will never become ill. You will be happy and you will never grieve.)