Dawkins is actually wrong about believers. They are not delusional, they are liars. He should have called his book 'The God Lie'.

A delusional person suffers from a mental disease(or is on psychotic drugs) that disables them from processing information properly. So they end up believing things that a normal functioning person can determine is not true.

Religious people know that the miracles and supernatural stories of the bible are pure myth. They lie and say they believe because of fear, pier pressure, because it makes them feel special and enables them to repress others.