First, do we even have the Koran that was written centuries ago? No, what we have is 347 ancient manuscripts that disagree with each other 97%. The modern Koran is simply pieces of each of these 347 manuscripts that have been pasted together.
Now, who wrote these 347 ancient manuscripts? These manuscripts were written by 691 different people, not by Muhammad. There is no evidence that Muhammad wrote the Koran or any part of the Koran. In fact, there is no evidence that Muhammad wrote anything. Now, worse than all of that, there is no proof that Muhammad ever existed. Islam is based on a lie, an imaginary book written by an imaginary person named “Muhammad.”