The only line of seperation is time. What about greek mythology? That used to be a true religion, even the romans adapted to it and made it their own. but now we consider it mythology. It can also go the other way round. For example: after people stopped believing in lycans, other people passed on the myths to their children about lycans. Eventualy, beleif of lycans was re-established.
The latter example was a bad one because a society of disbeliever also formed due to this.