Externally, religious groups are often discriminated against in a way that ghost-believers, for example, are not. Similarly to other categorizations (e.g. race, sexual orientation), the degree of sensitivity demanded often correlates to the degree of discrimination a given group has experienced.

Internally, religious people often consider their religion to be at the core of their identity in a way that ghost-believers do not. There often is also a sense of immutability to one's religion, and perhaps criticizing someone's inherent characteristics isn't fair game. Some religions believe this explicitly. Orthodox Judaism, for example, believes that once you're Jewish there is nothing you can ever do to change that, even if you claim to forsake Judaism or convert to another religion. It's supposedly an immutable part of your soul.

These two factors play off each other. Because believing in ghosts isn't at the core of someone's identity, they don't wear it on their sleeve and don't form large and influential social groups based around believing in ghosts. That means that external groups probably won't feel threatened by or suspicious of ghost-believers, and there probably won't be discrimination against them.

Of course, simply referring to someone as a "ghost-believer" suggests that they're not involved enough with that belief to warrant referring to it as a religious belief. There are no doubt some religions that believe in ghosts.

-- DanKarmel - 31 Jul 2010