Indeed, in the alternation of the night and the day and [in] what Allāh has created in the heavens and the earth are signs for a people who fear Allāh.
Allah tells us about the signs He created that are indicative of His complete power and great might. He made the rays that come forth from the bright sun as the source of light, and made the beams that come forth from the moon as light. He made them of two different natures so they would not be confused with one another. Allah made the dominion of the sun in the daytime and the moon in the night. He ordained phases for the moon, where it starts small then its light increases until it completes a full moon. Then it begins to decrease until it returns to its first phase at the conclusion of the month. Allah said:
Similarly, it was said in the second verse (6) that there are signs in the coming of the night after the day and the day after the night, and in everything Allah has created in the heavens and the earth. These are open proofs (of the Oneness of Allah and the inevitability of the Hereafter) for those who fear Allah.
That they are proofs of the Oneness of Allah (Tauhid) is not difficult to see. He has the power to make things happen. He makes things as no one can. He creates everything without the help of materials, personnel and prototype, and has the mastery to operate them under a system that never breaks or must be replaced.
And they are proofs of the 'Akhirah (Hereafter) because Allah is a great creator. He created, in His infinite wisdom, objects of such pro-portion and efficiency for the benefit of human beings. Then, He made them follow a remarkably stable system. Now, it is not possible that He would have created human beings, who are the served ones in this universe, just for nothing, or just to eat and drink, or in some other nonchalant manner without assigning any duties to them. When it stands settled that some restrictions must apply to the served one of this universe, it also becomes necessary that those who observe or do not observe these restrictions should face an accounting of what they have done, sometime, somewhere. Naturally, those who do observe these shall be rewarded and those who do not shall be punished. Then, it is also obvious that things do not work like this in the present world of our experience. Here, reward and punishment do not follow that pattern. It is quite likely that a criminal here may be living better as compared to someone God-fearing, righteous and innocent. Therefore, it is necessary to have a day for the accounting of deeds, rewards and punishments. This is what Qiyamah (The Last Day) and 'Akhirah (Hereafter) are.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 6–7
God has revealed the realities of the Hereafter by means of symbols. Similarly, in the present world, God reveals Himself in the form of arguments, and not as a tangible manifestation. So, we can find God only in the manner and shape in which He reveals Himself and not in any other form. In this world, God has offered mankind His guidance. But only those who are ready to follow it according to the godly plan, are destined to receive this guidance. Only those who are ready to understand the truth presented in the language of reason and accept it will be inspired to tread the right path. Those who do not bow down before true argument fail to bow down, as it were, before God; it is like not accepting God. Such people may expect nothing but Hell as their reward. In the heavens and on the earth, there are countless signs which become the elements of a lesson for those who are God-fearing. Fear, or apprehension, is a thing which makes a man serious. Unless a man is serious, he will not pay full attention to matters of importance and will not understand their various aspects. The whole universe is held together in a creative balance. This is a clear indication of the fact that the Lord of the universe is One who has absolute power over man. Similarly, the initial life which we are now experiencing provides clear proof of the fact that a second life is also possible. The appearance of material results in the present world and the non-appearance of moral results warrants the taking shape of another world where the moral results will be clearly in evidence. This is an unassailable truth, but this can be grasped only by one who looks at life in fear and apprehension.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.