The second verse (91) has been revealed in answer to those who had the audacity to say that Allah Ta` ala has just not revealed any book to any human being. According to them, this whole proposition of books and messengers was all wrong.
If, those who said that are the idolaters of Makkah, as mentioned by Ibn Kathir, then, the matter is obvious, for they did not believe in any book and prophet. And if they were Jews, as held by other commentators - and the contextual elements of the verse obviously support this view - then, their saying so was an outcome of their anger and irritation, something contrary to the dictates of their own religion. According to a narration of Imam al-Baghawi, this was the reason why the Jews too were displeased with the person who had made this unfortunate remark, and it was because of this erroneous conduct of his that he was removed from his office of religious authority.
In this verse, Allah Ta` ala has said to the Holy Prophet ﷺ that people who made this absurd remark did not recognize the Most True Allah as was His right to be recognized. Had they done that, this audacious remark would have never escaped their lips. Then, Allah Ta` ala tells the Holy Prophet ﷺ to ask the people who deny Scriptures absolutely: If that is what you think - that Allah Ta` ala did not reveal any book to any human being - then, you better have an answer as to who revealed the Torah, in which you too believe, and because of which you sit in judgment over your people. Along with it, they were also told: You are such connoisseurs of crooked ways that you would treat your own book, the Torah, which you believe in as being Holy Scripture, in a way that you got it conveniently written on separate sheets, rather than having it in a formally bound book, so that you can take out any sheet from it as and when you want it, and by doing so you would have the option of denying what it enjoined. An example of this are the verses of the Torah which described the signs and attributes of the Messenger of Allah which you have excluded. The last sentence of the verse: تَجْعَلُونَهُ قَرَاطِيسَ (which you kept in sheets) means exactly this. The word: قَرَاطِيسَ (qaratis) is the plural of: قِرطَاس (qirtas) which means sheet or paper.
After that, addressing the same people, it has been said: وَعُلِّمْتُم مَّا لَمْ تَعْلَمُوا أَنتُمْ وَلَا آبَاؤُكُمْ that is, you were taught (through the Qur an) what you or your forefathers did not know (earlier, either through the Torah or the Injil).
At the end of verse (91), it was said: قُلِ اللَّـهُ ۖ ثُمَّ ذَرْهُمْ فِي خَوْضِهِمْ يَلْعَبُونَ (Say, "Allah." Then leave them playing with what they are indulged in). In other words, the sense is: As for the question - if Allah just did not send any book, who sent the Torah? - it was something they would not know how to answer, so, you go ahead and tell them that it was revealed by none but Allah. Now that the argument against them stands concluded, your mission as far as they are concerned is over. So, let them be lost in the game they are playing.
Allah says that those who rejected His Messengers did not give Allah due consideration. Ibn `Abbas, Mujahid and `Abdullah bin Kathir said that this Ayah was revealed about the Quraysh. It was also said that it was revealed about some Jews.
(They said: "Nothing did Allah send down to any human being (by inspiration).") Allah also, said,
(Is it a wonder for mankind that We have inspired to a man from among themselves (saying): "Warn mankind.") 10:2, and,
(And nothing prevented men from believing when the guidance came to them, except that they said: "Has Allah sent a man as Messenger" Say: "If there were on the earth, angels walking about in peace and security, We should certainly have sent down for them from the heaven an angel as a Messenger.") 17:94-95. Allah said here,
(They did not estimate Allah with an estimation due to Him when they said: "Nothing did Allah send down to any human being (by inspiration).") Allah answered them,
(Say : "Who then sent down the Book which Musa brought, a light and a guidance to mankind") meaning, say, O Muhammad , to those who deny the concept that Allah sent down Books by revelation, answering them specifically,
(Who then sent down the Book which Musa brought) in reference to the Tawrah that you and all others know that Allah sent down to Musa, son of `Imran. Allah sent the Tawrah as a light and a guidance for people, so that it could shed light on the answers to various disputes, and to guide away from the darkness of doubts. Allah's statement, .
(which you have made into (separate) papersheets, disclosing (some of it) and concealing (much).) means, you made the Tawrah into separate sheets which you copied from the original and altered, changed and distorted as you wished. You then said, "this is from Allah," meaning it is in the revealed Book of Allah, when in fact, it is not from Allah. This is why Allah said here,
(which you have made into (separate) papersheets, disclosing (some of it) and concealing (much).) Allah said;
(And you were taught that which neither you nor your fathers knew.) meaning, Who sent down the Qur'an in which Allah taught you the news of those who were before you and the news of what will come after, that neither you nor your fathers had knowledge of. Allah's statement,
(Say: "Allah.") `Ali bin Abi Talhah reported that Ibn `Abbas said, "Meaning, `Say, Allah sent it down."' Allah said,
(Then leave them to play in their vain discussions.) leave them to play in ignorance and misguidance until the true news comes to them from Allah. Then, they will know whether the good end is theirs or for the fearful servants of Allah. Allah said,
(And this is a Book,) the Qur'an,
(Blessed, which We have sent down, confirming which came before it, so that you may warn the Mother of Towns) that is, Makkah,
(and all those around it...) refering to the Arabs and the rest of the children of Adam, Arabs and non-Arabs alike. Allah said in other Ayat,
(Say: "O mankind! Verily, I am sent to you all as the Messenger of Allah.") 7:158, and
("that I may therewith warn you and whomsoever it may reach.") 6:19, and
(but those of the sects who reject it, the Fire will be their promised meeting place) 11:17 and,
(Blessed be He Who sent down the criterion to His servant that he may be a warner to the `Alamin (mankind and Jinn).) 25:1, and,
(And say to those who were given the Scripture and to those who are illiterates: "Do you submit yourselves" If they do, they are rightly guided; but if they turn away, your duty is only to convey the Message; and Allah is All-Seer of (His) servants.) 3:20. It is recorded in the Two Sahihs, that the Messenger of Allah said,
(I have been given five things which were not given to any one else before me.) The Prophet mentioned among these five things,
(Every Prophet was sent only to his nation, but I have been sent to all people.) This is why Allah said,
(Those who believe in the Hereafter believe in it,) meaning, those who believe in Allah and the Last Day, believe in this blessed Book, the Qur'an, which We revealed to you, O Muhammad ,
(and they are constant in guarding their Salah.) for they perform what Allah ordered them, offering the prayers perfectly and on time.