That´s a false premise, Satan could just as well exist on his own as God or Santa or a unicorn...You are saying if the bible or koran or torah etc. is wrong about one thing (God) they are wrong about everything (Incl. Satan). Conversely if they are right about God they are right about everything, but obviously these religions (theories) could just as well only be right about one thing (Satan) and wrong about everything else. In fact most theories only explains parts of reality cos a theory that explains everything, in general explains nothing.