Science fiction religions

There have been several new religious movements based on science fictional religions. Wikipedia has a list of fictional religions, not all of which come from science fiction. The most famous example of a real-world recreation of a science fiction religion is the Jedi from Star Wars, but there is also the Church of All Worlds from Heinlein's Stranger in a Strange Land, Cullenism, based on Twilight. Recently I came across a recreation of the Bene Gesserit from Dune with its own training manual (PDF). I have no idea how serious this actually is but it looks as if it is an attempt to create a serious BG order. I think what all these new religious movements have in common (apart from the Church of All Worlds) is that they try to be too prescriptive about what people should practice and/or believe. On the other hand, the Bene Gesserit manual admonishes its readers:

“Beware of manuals! Manuals create habits!”

Except for the preliminary teachings tailored to the acolytes and postulants, we try to avoid admonitory sayings, but since this is our first edition and we must break the virgin soil, you will forgive the many errors inherent in this work. Someone had to do the plowing. Do not argue over the possible meanings of the contents of this manual. Words are dead things. Truth changes. Facts are fragile. Be Warned. Understand nothing. All comprehension is temporary. We realize, however, that a foundation is necessary, no matter how impermanent it may be. This is a real manual for real Bene Gesserit. It is not a guide book for children and their role-playing games. This is a guidebook for strong women to do great things.

Why do we have manuals? Answer: To disprove them.

Yes indeed - but then why write a manual at all?