So you believe that it's possible for a human being to live a completely sinless life at all times? That a human can never lie (even the slightest exaggeration is a lie), never be angry without just cause, never lust after anything, never be rebellious, never think incorrect thoughts etc? If so then you know nothing about human nature.
Mankind is fatally flawed. We are born into a sin nature as can be seen by the smallest of children instinctively lying. Surely God didn't create us this way as all that He does is good. Therefore we fell somewhere and let sin into our nature. For all have sinned.
Seems like the mohammedans have a poor logical grasp of reality if they missed something this basic.
GSA(P)