The world is a bouquet of diverse and varied beliefs and creeds. And some of them are mental. We define what belief is crazy by holding it against our own standards of sanity and legitimacy, but sometimes throughout the course of human history spiritual organisations have come along that are just inherently weird – and have been considered by the world at large to be cults. And maybe the term cult is like Potter Stewart's famous definition of pornography: “I know it when I see it.” Not something easily defined but obvious on inspection.
The study of religion is a fascinating one: many cultures and people throughout history have subscribed to an extraordinary array of belief systems, and it's not really the job of any one person to define the legitimacy or otherwise of anyone else's faith.
As a general rule we should try and be accepting and understanding as possible to different beliefs. There is too much conflict in the world based around religious disagreement and it's a lack of understanding and patience that fuel these conflicts. Conversely, if your belief system revolves around extraterrestrial beings, extortion, kidnapping, murder or divination through the toes then maybe you should have a good long look at yourself before looking beyond the confines of this world to another dimension where God is on the Jedi council.