Allah the Exalted emphasizes the greatness of the Qur'an, its high status and of being worthy of making hearts humble and rent asunder upon hearing it, because of the true promises and sure threats that it contains,
(Had We sent down this Qur'an on a mountain, you would surely have seen it humbling itself and rent asunder by the fear of Allah.) If this is the case with a mountain which is hard and huge, that if it was made able to comprehend and understand this Qur'an, will feel humble and crumble from fear of Allah the Exalted, then what about you -- O mankind Why do your hearts not feel softness and humbleness from the fear of Allah, even though you understand Allah's command and comprehend His Book This is why Allah said,
Having urged the people to be concerned about the Hereafter and having asserted the greatness of the Qur'an, this Surah is concluded with a few of Allah's attributes of perfection, as follows:
الِمُ الْغَيْبِ وَالشَّهَادَةِ (...the Knower of the unseen and the seen - 22). The attribute 'the Knower of the Unseen and the seen' means that Allah knows all things, visible or invisible.
Tazkirul Quran
Maulana Wahiduddin KhanOn ayahs 20–23
Human life is divided into two parts: ‘today’ and ‘tomorrow’. The present world is a man’s ‘today’ and the world of the Hereafter is man’s ‘tomorrow’. Whatever a man does in the present world, he has to face the basic consequences in the ensuing longer life. This is the truth and the other name for it is Islam. Man’s success depends upon his always keeping this reality in mind. The whole life of one who is forgetful of this, will go wrong. In this matter, there is no difference between a believer or a non-believer. Believers will have the advantage only if they acknowledge realities. If they become forgetful of them, they will also meet the same fate as their precursors.
Uthmani text of the Madinah mushaf (King Fahd Complex). Translation: Saheeh International. Tafsir via Quran.com.