Who really cares what Mohammed thought about the authenticity of Christian and Jewish Scriptures? I’m supposed to believe the testimony of some guy in the middle of Arabia, far from Jerusalem and 6 centuries removed from the events of the Gospels OVER the eye-witnesses of the events themselves? Why? Just because he said he saw an angel that no one else but him could see and that he made trips to heaven that no-one else but him could see and that that angel gave him the verses he recited to the Arabs? No thanks.
I’ll take the word of St. Peter, St. John and the others any day, than the word of a guy who had absolutely zero knowledge or connection in history, time or geography to the events of the Gospels.