Lots of questions, so this will take a bit.
The term scientology can refer to EITHER of two DIFFERENT things:
A. A spiritual practice which assists in developing insight & awareness of self & others originally developed by L.Ron Hubbard & numerous collaborators in the 1950s-1960s.
B. A cultish religion & church (Co$) established by L.Ron Hubbard renown for the abuses perpertrated on members, staff, & critics by church management.
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Why does it target celebreties?:
The subject of scientology can be of great assistance to individuals in increasing personal insight & awareness. As a result lots of performers & artists have found it useful in their creative work.
The church of scientology (Co$) has long used that interest by artists as a means of promoting the church to artists & their enthusiasts.
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What are Thetans?:
"Thetan" is simply another term for "spirit".
L.Ron Hubbard commonly invented or adapted words giving them specific new meanings in his lectures on the subject of scientology auditing.
As a result scientologists have their own slang for many concepts which occur in other spiritual traditions. "Thetan" is one such term, as are "ARC", "overt", & "serfac".
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Do they really believe in aliens?:
Neither yes, nor no.
Scientology is not a system of belief and does not have any teachings which are to be taken on faith as revealed truths. This would include a "belief" in aliens.
If you accept that man is truly an immortal spiritual being, as many scientologists come to do, then you might wonder what were these immortal spiritual beings doing before life on earth evolved to its present state.
One perspective is that these immortal spiritual beings had lifetimes (aka reincarnation) on other worlds & universes.
This is a belief, which is not fundamental or even universal within scientology. Many scientologists, however, DO consider it to be a distinct possibility.
It is a belief which is also commonly held in such other traditions as Hinduism, Buddhism, Taoism, and Gnosticism, among many other religious or philosophical schools.
Some aspects of scientology counselling seek to address an individual's private considerations on such matters when and if they arise.
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What's up with the personality tests and stuff?:
Scientology has observed that a person's mood often reflects their state of mind & spirit. Scientology practices can cause a person to experience improvements in spiritual state which are reflected in their mood patterns.
The personality tests were originally developed as a measuring tool to go along with the practice of auditing.
At some point, some "genius" in the church got the "bright idea" to use the tests as a means of marketing scientology to the general public. Since that time a great deal of waste paper has been generated all in the effort to hand out "free personality tests".
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What is Scientology?:
General Principles of Scientology:
The religion of scientology is perhaps better called a "spiritual practice" than a religion.
Belief is not really a factor in scientology, individuals are encouraged to explore their own truths and understandings of things. But I'll list the basic principles which underly the practice of scientology for your benefit.
Among the basic principles of scientology are the following:
A. Man is in essence an immortal spirit.
B. Human life represents a composite of a mortal body with the immortal spirit.
C. An individual is responsible for his own thoughts & actions.
D. An individual's thoughts & decisions may have unintended consequences including those of which he may not be aware.
E. Communication resolves problems and allows an individual to alter their thoughts & decisions.
F. Scientology focuses on techniques of communication and improvement of an individual's self-awareness.
Reincarnation is accepted as a possibility by scientology but it is NOT a belief which is taught in scientology. Individuals are free to accept or reject the prospect of reincarnation. Although most scientologists ultimately come to a personal acceptance of some aspect of reincarnation as a result of a better understanding of their own personal history of decision-making.
In short the principles of scientology are very similar in many ways to those of Buddhism.
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I am a freezone scientologist. I am not affiliated with the Church of Scientology.
Freezone Scientology practices scientology independent of the church of scientology.