Surah Aal-i-Imraan (3) — Ayah 4

4. 1. This book confirms the books that were revealed to the previous prophets, meaning it acknowledges the truth of what was recorded in them and the prophecies mentioned therein, which clearly means that this Qur'an is also revealed by the same Being who sent down many books before. If it had been from someone else or the result of human efforts, there would have been contradiction instead of consistency among them.

4. 2. That is, at their time, the Torah and the Gospel were certainly a source of guidance for people because that was the very purpose of their revelation. However, after this, it is clarified again by saying that now the era of the Torah and the Gospel has ended, now the Qur'an has been revealed, it is the Furqan, and now only it is the criterion for distinguishing truth from falsehood. Without believing in it as true, no one is a Muslim or a believer in the sight of Allah.

4. And (after them) He sent down the Criterion [3] (the Qur'an), which distinguishes between right and wrong. Those who reject Allah's revelations will have a severe [4] punishment, and Allah is Mighty, capable of retribution.

[3]
How is the Quran a Criterion (Furqan):

The earlier books include the Torah, the Psalms, the Gospel, the Scrolls of Adam, the Scrolls of Abraham, and the Scrolls of Moses, and this book (the Noble Quran) confirms that the aforementioned books are indeed revealed from Allah. By confirmation, it is by no means intended that all the material found in today’s Bible is revealed from Allah, because it is established from the Noble Quran itself that the People of the Book, whether Jews or Christians, have altered their books. By Torah is meant either the Ten Commandments that were given to Prophet Musa (Moses) ؑ in the form of tablets, or that divinely revealed revelation which was sent down to Prophet Musa ؑ. But in the present Old Testament of the Bible, which is called the Torah, much additional material has been included besides this. The Gospel, in reality, is the name of the collection of the speeches and admonitions of Prophet Isa (Jesus) ؑ, which he used to convey to people in the light of divinely revealed revelation. Since the Gospel was compiled by several of his disciples long after he was raised to the heavens, there are severe differences in it, and much more material has been added to its contents. That is why the Messenger of Allah ﷺ instructed the Muslims regarding those matters about which the Noble Quran is silent, that you should neither confirm nor deny the People of the Book. [بخاري، كتاب التفسير، باب __QUR_0__ ]
As for those matters which are against the Quran, there is no doubt about their being false, because it is not possible for there to be differences in the fundamental matters of divine revelation, and this is what is meant by the Quran being a criterion between truth and falsehood. And today, we can only certainly call that part of the Bible revealed from Allah which is in accordance with the Quran, and this is the meaning of believing in the previous revealed books.

[4] This punishment has already been given in this world and will also be given in the Hereafter. In this world, it happened in such a way that during his lifetime, Allah Almighty made Islam dominant, and all the opponents of Islam—whether they were polytheists or hypocrites, Jews or Christians—were humiliated and disgraced, killed, taken captive, exiled, or made to pay jizyah. As for the punishment of the Hereafter, it has been clarified that Allah Almighty is Mighty, Avenger, meaning He has full power to take retribution, and He will certainly take retribution from them.