Scientology isn't a religion. At all.
That's like saying... people who believe in The Force (from Star Wars) are following a valid religion. It's absurd. I personally thing all religions are equally as absurd, but.. that's for a different thread.
L. Ron Hubbard wrote some sci-fi books and thought it would be clever to try and convince people that certain events actually happened. How can someone sit there with a serious face and think there isn't anything wrong with that? If I sat here and made up a story of how a monkey named Charles gave me a holy banana on a desolate island and told you I believed him to be my god, would you look at me with a straight face? Maybe in principle you'd feel like saying, "If that's what you want to worship, so be it," but no. I would be a fuckin loon. That is... if I ACTUALLY believed in that.
Really, if you don't know the "story" of Scientology, read up on it. It's pretty f'n hilarious. If I recall properly, something about a intergalactic nuclear war billions of years ago that destroyed the earth. The beings that were on the earth were destroyed and split into tiny particles called (and I may be off on the name of this) "Thetas" that make up our being today.
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OK, got this backwards. "Body thetans" are spirits of aliens that inhibit the earth after the intergalactic war some billions of years ago. They cause you to do bad things.. give certain addictions, diseases, etc. Scientologists are on a mission to destroy these thetans.
More info here:
http://www.holysmoke.org/cos/body-thetans-def.htm
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These tiny 'thetas' make up our every thought, action, etc... and can be manipulated to your will only through the teachings of Scientology.
Still sound like a valid religion to you?
I don't really care who has what religion, but... when a celeb joins these religions, it will get a, "You GOTTA be fuckin kidding me... how stupid and gullible can people be?"
Hm, but honestly, if I was a celebrity, I would totally join Scientology just to play with the media. However, I think Cruise, Travolta... whoever, actually think Scientology to be something valid, which... I find quite hilarious.