simply put god needs beleivers to exist. if no one beleives in god there can be no god. take any other religion and you will find that to the followers of that religion their god(s) is just as real to them as god is to the christians.
why do we consider the gods of the greeks, romans, egyptians, mayans, etc. etc. nothing more than myth? because they are no longer beleived in or worshipped.