At least theoretically, they must exist, there must be a definition of "theist," otherwise the term "theist" is useless.
Surely St Anselm's "proof" applies to theists as much as it does to god. The ideal theist exists, as does the ideal atheist. Please Register or Log in to view the hidden image!
My point is mundanely linguistic: just like we have some idea of what a "table" is, or an "apple," or a "doctor," it is only reasonable that we also have some idea of what a "theist" is. Otherwise, we couldn't use that word.