The hypocrites fear lest a Surah should be revealed about them, showing them what is in their hearts. Say: "(Go ahead and) mock! But certainly Allah will bring to light all that you fear. (64)
The Hypocrites fear Public Exposure of Their Secrets
Mujahid said, "The hypocrites would say something to each other then declare, 'We wish that Allah does not expose this secret of ours," There is a similar Ayah to this one, that is, Allah's statement,
وَإِذَا جَاءُوكَ حَيَّوْكَ بِمَا لَمْ يُحَيِّكَ بِهِ اللَّهُ وَيَقُولُونَ فِي أَنفُسِهِمْ لَوْلَا يُعَذِّبُنَا اللَّهُ بِمَا نَقُولُ ۚ حَسْبُهُمْ جَهَنَّمُ يَصْلَوْنَهَا ۖ فَبِئْسَ الْمَصِيرُ
(And when they come to you, they greet you with a greeting wherewith Allah greets you not, and say within themselves: "Why should Allah punish us not for what we say?" Hell will be sufficient for them; they will burn therein. And worst indeed is that destination!)[58:8]. Allah said in this Ayah,
قُلِ اسْتَهْزِئُوا إِنَّ اللَّهَ مُخْرِجٌ مَّا تَحْذَرُونَ
(Say: "(Go ahead and) mock! But certainly Allah will bring to light all that you fear."),
He will expose and explain your reality to His Messenger ﷺ through revelation. Allah said in other Ayat,
أَمْ حَسِبَ الَّذِينَ فِي قُلُوبِهِم مَّرَضٌ أَن لَّن يُخْرِجَ اللَّهُ أَضْغَانَهُمْ
(Or do those in whose hearts is a disease (of hypocrisy), think that Allah will not bring to light all their hidden ill-wills?)[47:29], until,
وَلَتَعْرِفَنَّهُمْ فِي لَحْنِ الْقَوْلِ
(but surely, you will know them by the tone of their speech!)[47:30].
This is why, according to Qatadah, this Surah is called 'Al-Fadihah' (the Exposing), because it exposed the hypocrites.