Scientology is a cult founded by a really bad Science-fiction author (Ron L. Hubbard).

It has all sorts of strange beliefs, but those aren't particularly important. It's what it tells its members to do that matters:

-You should do everything in your power to distance yourself from family or friends.
-They have, at least once, tried to frame a critic by making it look like she was issuing bomb threats.
-Harassment and litigation of anyone who offends them. (Harassment such as firebombs, obscene phone calls, spreading rumours to their bosses, etc.)
-They've committed burglary to try and steal government documents critical of them.
-They're strongly opposed to all forms of psychology, calling it "abuse".

There are other, less substantiated claims, but those alone should give you pause.

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"Science is the poetry of reality." ~ Richard Dawkins