The Quran openly declares:
You (O Muhammad) was not a reader of any Scripture before it, nor did you write (such a Scripture) with your right hand, for then those who follow falsehood might (have a right) to doubt it. (al-Ankabut, 29.48)
It is an established, undeniable fact that the Prophet Muhammad, upon him be peace and blessings, was unlettered. Whereas the Quran, which he brought, has challenged all mankind with all their literary geniuses and scientists, from the first day of its revelation to the Last Day, to produce a like of it or even a single chapter of it:
If you are in doubt concerning that which We have sent down onto Our servant (Muhammad), then produce a chapter of the like thereof, and call your witnesses, supporters, who are apart from God, if you are truthful. (al-Baqara, 2.23)
Mankind have since been unable to produce a like of only one of its chapters, including, of course, its shortest ones like sura al-Ikhlas or sura al-Kawthar; those who have ventured to do that have all laid themselves open to ridicule. This is a clear proof for the Divine authorship of the Quran.