أَمْ أَمِنتُم مَّن فِي السَّمَاءِ أَن يُرْسِلَ عَلَيْكُمْ حَاصِبًا ۖ فَسَتَعْلَمُونَ كَيْفَ نَذِيرِ (Or, have you become fearless of Him who is in the sky if He looses a violent wind with stones against you? So, you will soon come to know how was My warning! - 67:17). In other words, they at the moment feel secure that the Supreme Being in the heaven will not release against them a sudden squall of stones together with violent wind. Soon they will realise how true the Divine warning was, but by the time they realise its veracity it would be too late. Now that they are safe, sound and secure, it is the time for them to give it a thought.
The next verse refers to the former nations who denied and rejected the truth and how they were annihilated by Divine scourge. Every person needs to draw a lesson from the punishment meted out to them.
وَلَقَدْ كَذَّبَ الَّذِينَ مِن قَبْلِهِمْ فَكَيْفَ كَانَ نَكِيرِ (And of course, those before them [ also ] rejected [ the truth ]. So, how was My censure?...67:18)
The next verse turns once again to the central theme of the Surah, that is, the description of various forms and functions of Allah's creation to prove His Oneness, His knowledge and power. Thus far human souls, forms and functions of the sky, the stars, the earth and so on were discussed. The next verse mentions birds that fly about in the atmosphere above us.