I think it's fair to describe them as a cult and not as a religion as they are really quite small. Small in this sense means a few things. Firstly membership, there are tens of thousands of Sceientologists2 compared to millions of even the smallest religions, (there are estimated to be almost 1 million Wiccans). Secondly this is a set of beliefs that is very young, L. Ron Hubbard made it up in 1952, compared to Islam, a very young religion at a merger 1400 years old. There's another factor too, it's a question of clubs. Micronations like Sealand or the Kingdom of Lovely are micronations because they are small, but also and more importantly, because real countries say they are. There are dozens of self-declared micronations out there that are larger in land and in population than Vatican City. Nationhood is about being a member of a club (in practice, of the UN) likewise being a religion means being respected as such and being invited to interfaith talks. (This is one possible argument why we could choose to not call atheism a religion). To me this is what we mean when we say a cult, we are thinking of a small, young, unaccepted religious group
So, I think it's fair to say that Scientology is a cult. But Sceientologists dont. Many have claimed that this is defamatory, and the word cult is derogatory in not uncertain terms. I want to work out why. The word cult is used about many strange and dangerous groups, the Jonestown Cult killed 400 of its own members, the branch Davidian cult fought a pitched battle with the American military during which 80 were killed. This image of the cult as having strange beliefs and an unhelpful obsession with violence etc is interesting. Because we know well that violence of the kind mentioned above is not unique or even more common in smaller faith groups, we also know that strange beliefs are not unique to such groups. (Look at it from an outsider's perspective, is it really more implausible that a galactic dictator brought billions of his people to volcanoes on earth and destroyed them with hydrogen bombs that that there are people alive who can cast a spell and literaly turn bread into the flesh of a 2000 year old dead jew).
So then if we can expect a cult to have violent elements and strange beliefs in ways similar to established religions, why is cult derogatory. There is a question of the club aspect of religions, many religious people (whilst clearly not agreeing with them) will respect the religion of another, but not of a cult member. I think there is an impression that cult beliefs are not very serious things. We dont think that anyone in their right mind can accept the beliefs of a cult. Tom Cruise is an international joke for being stupid enough to be part of a cult, often mocked by people whose own religion (if interpreted as literally as most interpret Scientology) contains elements just as foolish. Now, I must be clear that I dont want to dismiss thousands of years over very good theology by many of the greatest minds produced by humanity in explaining the strange beliefs in religions to make them more reasonable. There are great systems of thought around the great religions that are far more impressive intellectual achievements than the science fiction written by one guy that makes up Scientology's corpus. But is is fair for me to say that the beliefs of the average belier are made better by a more impressive theology? No, theology is not religion. Most religious people do not accept the version of religion presented by theology.
Is there a case for talking about liberal verses orthodox? For example, most Christians (in the UK at least) dont accept transubstantiation to be literally true (many do of course). But we expect that all Scientologist accept the Xenu story to be literally true. This is of course utterly false, for a start, the Xenu story is not told to the lower ranks at all, only the high level Tetans are expected to believe this, and how many of them believe it to be literally true and how many a good metaphor we cannot know. It's also important to consider how much the dogmas matter in a cult (matter of course to an outsider as we are considering defamation), it is assumed that pointing out how utterly insane one would have to be to believe in Xenu is enough to dismiss Scientology, if we do the same with transubstantiation then we are swiftly told "that's not the point". We think that ethics, or a relationship with God, or a sense of community are the key aspects of a religion, cults are often dismissed by pointing out that they hold insane doctrines.
There may well be a case that cult is simply a derogatory term because it contains all the negative qualities of religion without the implied respect for belief. After all, religions have a long track record of good deeds that we should (I mean this genuinely) respect and admire and it is assumed that cults do not. Religions help people out in times of trouble, and it is assumed that the beliefs of a cult cannot do this. Religions also present a strong tie to a community with shared heritage, this is not to say of course that cults cannot link us to a community, but the link to cultural heritage does not exist in a cult.
Scientology is a cult. We think this is a bad thing because the word cult presents a religion with all the stupidity and evil implied by religion but which we assume cannot have the redeeming qualities that religions have. I'm not sure the word really deserves the defamatory sting it has acquired. There are many cults (like Scientology) that are dangerous and evil, but the fact that they are cults does not imply this any more than is implied by being part of a religion.
1) What that word means is utterly beyond me, can anyone help? I've never seen a definition that wasn't “stuff that's ultimate or beyond reality”, well, like what for instance?
2) This is disputed by Scientologist who claim there are anything up to 10 million of them, but if so they're very reluctant to say so in surveys of religious belief and on the census.