Like I said, you really have no idea how it was collated and written down, your knowledge of history for this region and period is very lacking. Christianity wasn't present in Mecca and Medina when Islam was born. Judaism was and the Jewish community was regarded as special and learned. Christians and Jews are ahl al-Kitab, people of the book, and are equal to Muslims, so there is no threat or challenge from these groups. Likewise the prophets are recognised too, as prophets.
Our knowledge of how the Koran and Hadith were collated and recorded is based on historical fact. Mohammed lived, there is no doubt of this. Whether the Koran is the word of god, well I can't answer that. Remember Muhammed lived from 570 to 632 CE. Our historical record is far more accurate because of this.
All you need to do is read a little, learn a little history!