And He it is Who gives life and causes death and the alternation of the night and the day is due to Him. Will you not then understand? (Al Quran, Surah Al Muminun, Chapter 23: Verse 80)
Following on from Chapter 23: Verses 68-74, (see: The Quran Asks 23-68 for details). The above verse shall be understood in context with the following verses:
23-75: And (even) if We show mercy to them and remove the harm with which they are afflicted, they would still persist blindly wandering on in their rebellion. (1)
23-76: And We had caught them in the punishment but (even then) they do not submit themselves to their Fosterer, and they do not humble themselves,
23-77: until when We open on them the door of severe punishment, then they will be in despair therein. (R 4)
23-78: And He it is Who has produced for you the ears and the eyes and the hearts (but) the gratitude which you show is (comparatively) less. (2)
23-79: And He it is Who has multiplied (and spread) you in the earth and to Him you will be gathered.
23-80: And He it is Who gives life and causes death and the alternation of the night and the day is due to Him. Will you not then understand?
Summarized Commentary:
(1) Whether Allah (SWT) shows mercy to the infidels or punishes them, they will not submit till they are severely punished when they will be hopeless.
(2) Allah (SWT) mentions some of His favours on mankind and His powers so that they should reflect upon them and exhibit gratitude and accept Him as the Supreme Authority.