Why the Quran mentions The Gospel, The Torah, the Bible

Christians often pose verses from the Qu'ran such as Surah 2:41 or Surah 4:41 that speak about the Qur'an confirming the previous scriptures and from that they conclude that the Qur'an is affirming the authenticity of these scriptures.

Thus we can see that the Qu'ran confirming the previous scriptures most reasonably indicates that the Qur'an is testifying to the truth of the previous revelations that spoke about the coming of Prophet Muhammad peace be upon him. 

Ibn Kathir stresses this in his commentary on Surah 3:3 (Source)

Imam Al Tabari stresses this in his commentary on Surah 2:101 (Source)

Imam Al Qurtubi stresses this in his commentary on Surah 46:12 (Source)

Muqaatil bin Sulaiman stresses this in his commentary on Surah 2:101 (Source)

Some have argued back that verses such as Surah 2:91 stress that the Qur'an is confirming the scriptures that is in the possession of the Christians and Jews at the time of the Prophet Muhammad peace be upon him and not simply the original revelations that God sent down in the past. However, Imam az-Zamakhshari clarifies...

The Qur'an claims to be a confirmation, protector and touchstone of the Truth contained in the Torah and Gospel. (Az-Zamakhshari, op. cit., vol.2, p.575, cited here)

Notice how az-Zamakhshari states that one of the purposes of the Qur'an is to confirm and protect the Truth contained in the Torah and Gospel. Now, if Zamakhshari believed that the Torah and the Gospel were pure and uncorrupted, he would have simply stated that the Qur'an confirms the entire Torah and Gospel in the possession of the Christians and Jews. However, he wanted to clarify what is meant by the Quran's confirmation and that is that it confirms the truth in the Torah and Gospel, which indicates that the Torah and Gospel contain falsehood. Thus, az-Zamakhshari makes it clear that he believes that the Torah and Gospels were textually distorted.   

Ibn Kathir also reaffirms this position...

(but a confirmation of that which was before it) in reference to the previously revealed Divine Books, by which this Qur'an testifies to the true parts that remain in them and denies and refutes the forged parts that were added, changed and falsified by people. The Qur'an accepts or abrogates whatever Allah wills of these Books, (Tafsir Ibn Kathir, Source)

Ibn Kathir also quotes well known Qur'anic commentator Ibn Jarir Al Tabari as saying...

Ibn Jarir said, "The Qur'an is trustworthy over the Books that preceded it. Therefore, whatever in these previous Books conforms to the Qur'an is true, and whatever disagrees with the Qur'an is false.'' (Tafsir Ibn Kathir, Source, You can also see Al Tabari saying this himself in his commentary on Surah 2:41)

Imam al-Tabari relates from the eminent Jurist Ibn Juraij (80-150 AH/699-767 CE) that if the people of the book quote something from their Bible that disagrees with the Qur'an then we reject it but if it agrees with the Qur'an then we accept it. (Source) Imam al-Bahgawi also quotes the opinion of Ibn Juraij in his commentary. (Tafsir Al Baghawi, 1/65) Abu al-Layth al-Samarqandi (d. 373 A.H.), the well known Hanafi scholar in his commentary of the Qur'an known as Bahr al-'Ulum on Surah 5, verse 48 says that the Qur'an acts as a judge over the previous scriptures. This indicates that the Qur'an judges whether something is true or false. (Source)

The Qur'an is the protector of the Gospel and Torah. But which Gospel and Torah? It is the original revelations sent to Jesus and Moses (peace be upon them both) and whatever of it survives today (e.g. prophecies of Muhammad peace be upon him to come) in the scriptures of the Christians and Jews today.

Thus, Christian missionaries who like to misinterpret what the verses are actually saying would need to do better and provide objective evidence that the verses they are posing do intend to communicate what they claim it is. If they cannot, then we must harmonize these verses with the Islamic teachings that make it clear that the previous scriptures have been textually corrupted.