A redated post.
If two statements contradict one another, one must be true and the other false. That is the most fundamental law of logic. Thus the claims:
1) "God' final revelation to man is in the Qu'ran, where it is taught that Jesus never died by crucifixion."
2) "Christ's death on the cross atones for our sins."
1 logically entails
1A) Jesus did not die by crucifixion.
While 2 logically entials
2A) Jesus died by crucifixion.
Thus the central claims of Islam and the central claims of Christianity cannot both be true. The issue, by the way, is a factual issue. It does not even involve the supernatural.