Most the members aren't allowed to know the mythology. You don't learn the mythology until you've been in for many years if you get the point where you are allowed to learn it at all. By the time you reach that point you are basically ready to believe anything they tell you. You have separated from all your non-scientologist family and friends, you have probably signed a bunch of questionable contracts and are deeply in debt or have paid tens of thousands of dollars (they don't teach lost cost fallacy). You have been told for many years about the incredible power and happiness of the people who rank above you in the process and your own lack of happiness makes you chase the idea that if you keep to it maybe it will happen to you. You feel like a fraud because so much of this stuff isn't happening to you and the solution is to try harder, believe harder.
Some of the mythology is common knowledge through the Book of Dianetics written by the founder of Scientology: http://en.wikipedia.org/wiki/Dianetics It has to start somewhere and Hubbard's sci-fi authorial past seems to do the job until this day.