Why should we believe the Quran if it comes 600 years after Jesus?
The Quran may have come 600 years after Jesus, but does that make it false? Jesus came 1600 years after Moses, does that make Jesus false? The underlying or main argument here is the time between one person and another, but as we can see, if time is a problem then Christians should reject the other Prophets and Messengers since they have a greater number of years between them.
The complete manuscript collections of the New Testament and Old Testament date to the 4th and 5th century, within 100 years of the Qur'an, if Christians want to reject the Qur'an because it is 600 years after Jesus then they must be consistent and also reject the New Testament.
It should be noted that the Church itself did not ordain or make it canon (law) that the New Testament should be scripture until 393 and 397 CE in the Carthaginian councils. With all of these evidences, and centuries between Jesus and the New Testament, then they must reject the New Testament first. If not, then they are admitting that their argument is weak, unintelligent and absurd.