Another point that I forgot to bring up in my first post is the tiered system of learning within Scientology. One has to work at the religion and learn the laws and truths in order to achieve an official higher rank within the religion that grants you deeper access within. I think this bodes in favour of cult rather than religion, if only in the levels of commitment and zealousness that this practice of ranking denotes. It also suggests a deeper level of organisation where one is working to get closer to the levels of understanding that the leader possesses. This is not found in religions, at least not to this organised and official extent.
What you're describing is, I believe, called a "cult of mystery" where the higher one rises within the religion or cult the closer to truth they supposedly get and the more knowledge they receive. It was a more common thing in the past, like in ancient Greek, Roman, and Near-East religions. It was probably more acceptable to say that truth is reserved for a select few back then since today lots of people would be put off by such an idea.
I'm not entirely sure, but I believe there were some Christian sects that were set up this way, like the Gnostics. I'd expect most religions had similar variations at one time or another.
But anyway, I'd agree that this kind of secrecy leans more toward cult than religion on the whole.