The Quran is no new heavenly scripture. It is only an original edition of the previous heavenly scriptures. The Quran enshrines the same teachings as found in previously revealed scriptures. However, these ancient scriptures are no longer preserved in their original state. Later additions and deletions have rendered them unreliable, whereas the Quran, preserved in its original state, is reliable. When a believer in the previously revealed scripture turns to the Quran, it does not mean that he rejects his own belief but instead amounts to his having rediscovered his faith in an original form. In this way, the Quran is the Guidebook for all humanity.

Source: Quran for All Humanity