It's no more or less believable than any other religion. Do you think people really believe that a dead guy came back to life? How about an entire ocean was suddenly parted so the good guys could get away and then collapsed again on the bad guys? Or that the earth is 6000 years old? Or that the guy who created the entire universe 12 billion years ago and billions of light years large is really really concerned about if human penises wind up in human vaginas before the correct ritual is performed?
You can sort-of understand why ancient religions came about and stuck. People in general, without education, before the formulation of the Scientific Method, living in a very uncertain world where starvation and disease were all about (and life was short and harsh) would invent supernatural explanations for things and perhaps like to believe that there was someone looking over them in judgement all the time.
However, there is absolutely no excuse for Scientology to be as big as it is. It was conceived as a cynical exercise in demonstrating that gullibility, ignorance and superstition, which are fundamental parts of human nature, are every present and easily exploitable (for money) and that society has not advanced to the point that the human race has outgrown its primitive cultural roots.
You have to hand it to L Ron, it was a dastardly,cynical plan to make money out of the stupid, and it has been a soaring success.
As they say, a fool and his money are easily parted. There are a lot of Hollywood actors involved...