The Prophets
لَوْ يَعْلَمُ ٱلَّذِينَ كَفَرُوا۟ حِينَ لَا يَكُفُّونَ عَن وُجُوهِهِمُ ٱلنَّارَ وَلَا عَن ظُهُورِهِمْ وَلَا هُمْ يُنصَرُونَ
If those who disbelieved but knew the time when they will not avert the Fire from their faces or from their backs and they will not be aided...
Nearby ayahs help place verse 21:39 in sequence and meaning.
خُلِقَ ٱلْإِنسَـٰنُ مِنْ عَجَلٍ ۚ سَأُو۟رِيكُمْ ءَايَـٰتِى فَلَا تَسْتَعْجِلُونِ
Man was created of haste. I will show you My signs, so do not impatiently urge Me.
وَيَقُولُونَ مَتَىٰ هَـٰذَا ٱلْوَعْدُ إِن كُنتُمْ صَـٰدِقِينَ
And they say, "When is this promise, if you should be truthful?"
بَلْ تَأْتِيهِم بَغْتَةً فَتَبْهَتُهُمْ فَلَا يَسْتَطِيعُونَ رَدَّهَا وَلَا هُمْ يُنظَرُونَ
Rather, it will come to them unexpectedly and bewilder them, and they will not be able to repel it, nor will they be reprieved.
وَلَقَدِ ٱسْتُهْزِئَ بِرُسُلٍ مِّن قَبْلِكَ فَحَاقَ بِٱلَّذِينَ سَخِرُوا۟ مِنْهُم مَّا كَانُوا۟ بِهِۦ يَسْتَهْزِءُونَ
And already were messengers ridiculed before you, but those who mocked them were enveloped by what they used to ridicule.