Muslims, who wrote the Quran?

You are making a lot of umptions there. "Clic Arabic" is a misnomer. It was dialect Arabic at the time of Muhammad and not a uniform written language. The so-called "beautiful poetry and prose" is actually a grammatical and incomprehensible mess in many parts of the Koran. It just doesn't make sense even in Arabic. The verses were indeed compiled much later after the prophet's death with the Uthmanic recension of the Koran. And even during the time of the prophet, we know that fragments were written by scribes upon chards, leaves and bone or upon whatever was available. But, the original sayings or so-called revelations of Muhammad were never a unitary literary work. And according to hadith traditions, some of it had been lost. The Koran itself is a hotch-potch of borrowed source material from Hebrew scripture, the Christian New Testament, late Christian apochryphal literature, Rabbinical Judaism, pre-Islamic Arabian legend and tradition, the Hanefites, as well as Persian and Greek sources. It also contains the stories and legends of the exploits of Muhammad with Muhammad's own injunctions. Like all Semitic languages, the Koran originally never had vowel pointing which caused confusion over the proper reading or even the intended meaning of many words. A thorough study of Source Criticism, Redaction Criticism, Form Criticism, Historico-literary Criticism and Textual Criticism will easily dismantle any of the Othodox Muslim claims regarding the origins of the Koran. Whenever a perfectly natural, rational explanation can be provided for any phenomenon, there is no reason to ume a supernatural one since supernatural agency is outside the realm of normal human experience. It is not rational to ume what is not common to normal human experience. Since there is nothing in the Koran or it origins that cannot otherwise be explained through human agency, there is no reason to ume a divine agency.