Abbas - Tanwîr al-Miqbâs min Tafsîr Ibn ‘Abbâs
(And (bethink you of) the day when We raise in every nation) when We take from every group (a witness) a prophet (against them) testifying that he conveyed the message to them (of their own folk) a human being like them, (and We bring thee) O Muhammad (as a witness against these) against your people; it is also said that this means: a witness for your nation that they had accepted the message. (And We reveal the Scripture unto thee) and We send Gabriel to you with the Qur'an (as an exposition of all things) of the lawful and unlawful, the commands and prohibitions, (and a guidance) from error (and a mercy) preventing from chastisement (and good tidings for those who have surrendered (to Allah)) that they will have Paradise.
And, mention, the day We shall raise up from every community a witness against them from among themselves, and this will be their [that community’s] prophet, and We shall bring you, O Muhammad (s), as a witness against these, namely, your people. And We have revealed to you the Book, the Qur’ān, as a clarification of all things, that people [might] need concerning the Law, and as a guidance, from error, and a mercy and good tidings, of Paradise, to those who submit, [those who] affirm the Oneness [of God].
and We shall bring you as a witness against these: this has already been mentioned in sūrat al-NisāÌ. And We have revealed to you the Book, namely, the Discriminating Intellect after the existence that is bestowed by the Truth, as a clarification of all things, explaining and verifying the truth of all things, and as a guidance to his perfection for whoever submits and yields to the soundness of his primordial nature, and a mercy, for him by making him attain that perfection by means of [spiritual] nurturing and reinforcement, and as good tidings to him that he will subsist in that perfection forever beyond time in the three gardens.
(And on the Day when We resurrect a witness from each nation from among themselves, and We bring you (O Muhammad) as a witness against these.), meaning, your Ummah. The Ayah means: remember that Day and its terrors, and the great honor and high position that Allah has bestowed upon you. This Ayah is like the Ayah with which `Abdullah bin Mas`ud ended when he recited to the Messenger of Allah from the beginning of Surat An-Nisa'. When he reached the Ayah:
(And We revealed the Book (the Qur'an) to you as an explanation of everything,) Ibn Mas`ud said: "Allah made it clear that in this Qur'an there is complete knowledge and about everything.'' The Qur'an contains all kinds of beneficial knowledge, such as reports of what happened in the past, information about what is yet to come, what is lawful and unlawful, and what people need to know about their worldly affairs, their religion, their livelihood in this world, and their destiny in the Hereafter.
(a guidance) means, for their hearts.
وَرَحْمَةً وَبُشْرَى لِلْمُسْلِمِينَ
(a mercy, and good news for the Muslims.) Al-Awza`i said:
(And We have revealed the Book (the Qur'an) as an explanation of everything,) meaning, with the Sunnah. The is the reason why the phrase,
وَنَزَّلْنَا عَلَيْكَ الْكِتَـبَ
(And We have revealed the Book to you) is mentioned immediately after the phrase,
وَجِئْنَا بِكَ شَهِيدًا عَلَى هَـؤُلآءِ
(And We shall bring you (O Muhammad) as a witness against these. ) the meaning - and Allah knows best - is that the One Who obligated you to convey the Book which He revealed to you, will also ask you about that on the Day of Resurrection.
(On the Day when Allah gathers the Messengers together and says to them: "What was the response you received (to your Message)'' They will say: "We have no knowledge, indeed only You are the Knower of all that is hidden.'') (5:109) And Allah says:
(Verily, He Who obligated the Qur'an upon you (O Muhammad) will surely bring you back to the return.) 28:85 meaning, the One Who gave you the obligation of conveying the Qur'an will bring you back to Him, and your return will be on the Day of Resurrection, and He will question you about you commission of the duty He gave you. This is one of the opinions, and it presents a good understanding of it.
Maududi - Sayyid Abul Ala Maududi - Tafhim al-Qur'an
The Qur'an makes manifest everything on which depends guidance or deviation, success or failure: whose knowledge is essential for following the Right Way: which clearly distinguishes Truth from false hood.
In this connection one should guard against that meaning of this sentence and the like in the Qur'an according to which some people interpret "everything" to mean "the knowledge of all sciences, arts, etc.," and in order to prove the correctness of their interpretation, they have to pervert the real meaning of the Qur'an.
This Book is a guidance for those who surrender to it as a Divine Book and follow it in every aspect of life. Then it will bring Allah's blessings upon them, and will give them the good news that they will come out successful in the Court of Allah on the Day of Judgment. Or. the contrary, those people who reject it shall not only be deprived of guidance and blessing but will also find it as a testimony against themselves on the Day of Resurrection when Allah's Messenger will stand up to testify against them. This Book will prove to be a strong argument against them, for Allah's Messenger will say that he had conveyed its Message which made manifest the distinction between Truth and falsehood.