Why do some christians say Muslims do not believe Jesus is the messiah?

they believe that mohammed or however it is spelled had a vision from the arc angel gabriel coming dow to earth bringing forth the quran...but muslim nation was born thousands of years after cristianity and those that study the bible know that the apostale paul said that even if an angel was to descend from the heavens and show you a religion you was not taught be him damned (Galatians 1:8),, and what do the muslims say. they even have a tomb dug up for jesus when he returns to get married and then die just to be buried side by side to a child molester called mohammed or whatever his name is.