Oh! Cultists? These guys arent unknown in the Aliens universe. The first cult rose up in Aliens Earth Hive, and the real reason why the xenos escaped in the first place. It was led by a guy named Salvaje. You see, in the early comics and stuff, xenos were able to communicate sorta, using telepathic dreams. People that recieved these dreams either saw them as horrible nightmares, or visions of 'The true Messiah'. Of course, it was only a way to lure people into being hosts, which worked quite well.
The place where the first xeno was being housed in Lima, which had matured into a queen and laying eggs, was raided by hundreds of cultists. Many gave themselves willingly as hosts, but some sorta regretted it. Before a destruct sequence was activated, around a dozen inpregnated hosts got free.
Anywhat, a lot of cults exist after the alien wars, but mostly through the spread of Royal Jelly which is in the extended universe. You can see quite a bit of this in the Omnibus series, mostly in the first and fourth volumes. Freaks...
Anywho, Im pretty darn sure the cultists in the game are only there to kinda give you a freebee, since its the last level and its the toughest. After all, you are going up against a full sized hive of the K-Series.
Hahahaha! Hope I didnt talk anyone's ears off with the whole thing with the extened universe.