Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And they) this refers to 'Abdullah Ibn Umayyah al-Makhzumi and his host who (say) to Muhammad (pbuh): (O thou unto whom the Reminder is revealed) O you who claim that Gabriel brought you the Qur'an, (lo! thou art indeed a madman!) who is inventing lies.
(And those who do not expect a meeting with Us (i. e., those who deny the Day of Resurrection and the life of the Hereafter), say: "Why are not the angels sent down to us, or why do we not see our Lord'' Indeed they think too highly of themselves, and are scornful with great pride. On the Day that they do see the angels - there will be no good news given on that day to the guilty. And they (angels) will say: "All kinds of glad tidings are forbidden for you.'') (25:21-22) For this reason Allah said:
(We do not send the angels down except with the truth, and in that case, they (the disbelievers) would have no respite!) Mujahid said in this Ayah:
مَا نُنَزِّلُ الْمَلَـئِكَةَ إِلاَّ بِالحَقِّ
(We do not send the angels down except with the truth) "i.e., with the Message and the punishment.'' Then Allah, may He be exalted, stated that He is the One Who revealed the Dhikr to him, which is the Qur'an, and He is protecting it from being changed or altered.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
The 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.
This 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)
"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.”
That 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."