This is a common question that many Christians pose to Muslims. They argue that the Qur’an cannot have been preserved since Allah swt failed to protect his earlier revelations like the Torah and Gospel (Injeel), so He has also been ineffective at protecting the Qur’an from corruption.
The Islamic belief is that all the Prophets from Adam (a.s) to Muhammad (pbuh) taught the same religion and stated the same principles for their community to believe (21:25). In other words, the common bond between them is that Allah swt alone is to be worshipped, with no partner or associate, only their laws differed (5:48).
The Qur’an has declared that it is not only uncorrupted, but also incorruptible, since no evil or falsehood can ever approach it from any direction in order to corrupt it: “And indeed, it is a mighty Book. Falsehood cannot approach it from before it or from behind it (41:41-42) and has not made therein any deviance.” (18:1) This conveys a warning concerning the authenticity and integrity of the texts of previous revelations.
Allah swt did not promise to preserve previous revelations because they were directed towards a particular community of people for a specific time only (16:36, 10:47, 35:24, 13:38), so they were entrusted with the scholars (5:44). Allah promised to protect the Qur’an (15:9, 41:42) from any corruption in a preserved tablet with Him (85:21-22).
He promised to preserve the Qur’an because it is the final revelation for all of mankind to obey until the end of time. Prophet Muhammad pbuh is a warner to the worlds (25:1, 6:19, 68:52, 38:86-87), and the final prophet for all ages and all people, not just a specific nation as was the case with other prophets.
The primary function of the Qur’an is not just to confirm truth that was sent down in previous revelations, but to also guard and protect that truth so that it can never be lost or distorted to remind mankind of Truth – which is belief in only One God – and complete Submission to Him.