Allah is informing us that He never destroys a township until He has established evidences for it and its allotted time has ended. When the time for a nation's destruction has come, He never delays it, and He never moves its appointed time forward. This was a message and a warning to the people of Makkah, telling them to give up their Shirk, their stubbornness and disbelief for which they deserved to be destroyed.

﴿وَقَالُواْ يأَيُّهَا الَّذِى نُزِّلَ عَلَيْهِ الذِّكْرُ إِنَّكَ لَمَجْنُونٌ - لَّوْ مَا تَأْتِينَا بِالْمَلَـئِكَةِ إِن كُنتَ مِنَ الصَّـدِقِينَ - مَا نُنَزِّلُ الْمَلَـئِكَةَ إِلاَّ بِالحَقِّ وَمَا كَانُواْ إِذًا مُّنظَرِينَ - إِنَّا نَحْنُ نَزَّلْنَا الذِّكْرَ وَإِنَّا لَهُ لَحَـفِظُونَ ﴾

(6. And they say: "O you (Muhammad) to whom the Dhikr (the Qur'an) has been revealed! Verily, you are a madman!) (7. "Why do you not bring angels to us if you are of the truthful'') (8. We do not send the angels down except with the truth, and in that case, they (the disbelievers) would have no respite!) (9. Verily, We, it is We Who revealed the Dhikr (i.e. the Qur'an) and surely We will guard it (from corruption).)