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Surah 15. Al-Hijr
Ayah 6 - 9
6 وَقَالُوا۟ يَـٰٓأَيُّهَا ٱلَّذِى نُزِّلَ عَلَيْهِ ٱلذِّكْرُ إِنَّكَ لَمَجْنُونٌ
7 لَّوْ مَا تَأْتِينَا بِٱلْمَلَـٰٓئِكَةِ إِن كُنتَ مِنَ ٱلصَّـٰدِقِينَ
8 مَا نُنَزِّلُ ٱلْمَلَـٰٓئِكَةَ إِلَّا بِٱلْحَقِّ وَمَا كَانُوٓا۟ إِذًا مُّنظَرِينَ
9 إِنَّا نَحْنُ نَزَّلْنَا ٱلذِّكْرَ وَإِنَّا لَهُۥ لَحَـٰفِظُونَ
[6-9] They say, "You, upon whom the Divine word3 has been sent down, are surely insane. Why do you not bring angels before us, if what you say be true?" We do not send down angels in this way. When they come down,4 they come down with truth, and then the people are not given respite.5 As regards this Admonition, it is We Who have sent it down, and We Ourself will preserve it.6
3The word (Zikr) literally means "to cause to remember, "to caution" and "to give advice". But the Qur'an has used it as a technical term for "Admonition", which comes as a precept. Thus all the Books that had been sent down to the Messengers were Zikr, and the Qur'an is also Zikr.
4This was a sarcastic remark that was made by the opponents, for they did not acknowledge that the "Admonition" had been sent down to the Holy Prophet: otherwise they could not say that he was "insane". What they really meant by this remark was: "You, who claim that the "Admonition" has been sent down to you, are insane". A similar remark was also made by Pharaoh concerning Prophet Moses. Addressing his courtiers, he said, "The Messenger who has been sent to you is insane". (XXVI: 27)
5"We do not send down the angels in this way": We do not send down the angels for the mere fun of it in response to the request of the people: nor are they sent to unveil the Reality before them in order to show to them all the "unseen" things to which the Messengers invite them to believe. As a matter of fact, angels are sent down on that occasion when it is decreed to pass judgment on sane wicked people. At that time the judgment comes into operation without extending. any invitation to the condemned people to accept the Message, for their period of respite ends as soon as the "Reality" is unveiled before them.
"they come down with truth" = "they bring down truth with them." That is they come down to eradicate falsehood and to establish truth in its stead. Or, in other words, it means, “they come down to put into force the judgment of Allah.”
6That is, "You should note it well that it is We Who have sent this `Word'. Thus it is not Our Messenger whom you are calling `insane' but in fact this abusive remark applies to Us. Moreover, you should know that it is Our Word and We are preserving it; therefore, you can do no harm to it; nor can you discredit it by your ridicules, taunts and objections: nor can you hamper its progress, whatever you may do against it: nor will anyone be ever able to change or tamper with it."