First, he was illiterate !! How can an illiterate person come up with such a rich, poetic, intellectual, and inspiring text that it rocked the entire Arabia?
Mohammad (pbuh) never went to school! No one taught him. He had no teacher of any kind in any subjects. How can he have the knowledge of all the science, astronomy, oceanography, etc that is contained in the Quran? (For example, the mention of ocean currents, stars, earth, moon, sun and their fixed paths in Soorah Rahman; and many other scientific statements that are found in Quran, that I cannot state in this short article)
When Quran was revealed, the Arabic language was at its peak in richness, poetic value, literature, etc. Quran came and challenged the best literature in Arabic, the best poetry in Arabic of the time. Mohammad (pbuh) being illiterate couldnt possibly have come up with something so immaculate that it even exceded the best of poetry, and literature in Arabic at the time of the language's PEAK development. Arabic language had never been so rich in expression, poetic value, vocabulary, and variety in literature, as it was in the time of Quran. At a time like this, Quran came and exceeded the best of Arabic in all aspects of the language: poetry, literature, expression, etc. Any classical Arabic speaker would appreciate the unbeatten, unchallenged, and unmatched beauty of the language of Quran.
An illiterate man is simply not capable of writing such a book.
Mohammad (pbuh) had no reason to come up with something like Quran, and cause the entire society of Arabia to become his enemy. Why would he do something like that? Why would he write something going against almost all of the norms of the society, and lose his family, relatives, friends, and other loved ones, and not to mention all the wealth he lost
Quran was revealed over a period of 23 years! A very long time! Is it possible for someone to maintain the same exact style of Arabic speech , as demonstrated in Quran, for over 23 years?
Also, what the prophet Mohammad (saaw) used to say is recorded in what we call his hadeeth (sunnah). If we look at the Arabic style of the hadeeth, and compare it with the style of Quran, we can clearly see that they are clearly DIFFERENT, and DISTINGUISHABLE Arabic styles. The prophet (saaw) spoke in public. It does not make sense that a man has two UNIQUE, Distinguishable, and completely different styles of speech in public. Yet another reason why Mohammad (saaw) couldn't possibly have written Quran.
Quran was revealed over a period of 23 years ! A very long time! Is it possible for someone to maintain the same exact style of Arabic speech , as demonstrated in Quran over 23 years?