(… أَمْ أَمِنتُم مَّن فِي السَّمَاءِ أَن يُرْسِلَ عَلَيْكُمْ حَاصِبًا) (Or do you feel secure that He who [holds authority] in the heaven would not send against you a storm of stones?…): The punishment of humans towards each other is according to their capabilities, whereas Allah’s punishment matches His power, so can we endure His punishment? Of course not:

 أَفَأَمِنتُمْ أَن يَخْسِفَ بِكُمْ جَانِبَ الْبَرِّ أَوْ يُرْسِلَ عَلَيْكُمْ حَاصِبًا ثُمَّ لَا تَجِدُوا لَكُمْ وَكِيلًا

(Then do you feel secure that [instead] He will not cause a part of the land to swallow you or send against you a storm of stones? Then you would not find for yourselves an advocate.) (Al-Israa: 68)

The (storm of stones) denotes small stones that are kind of punishment which no one can resist, so that Allah (swt) says (Then you would not find for yourselves an advocate).

(  فَسَتَعْلَمُونَ كَيْفَ نَذِيرِ… ) (…Then you would know how [severe] was My warning): Allah (swt) is addressing the people of Mecca saying “You will realize how serious the consequences of My warning once punishment befalls upon you”.