The Campaign Builder's Guild

My GURPS campaign makes a lot of use of superstition.  Pretty much anything supernatural or magical is false, with the explanations ranging between coincidence and outright "that's what it does anyways".  An orange doesn't magically cure scurvy, it's giving you Vitamin C.  A balm of turpentine, vinegar, egg white and rose oil doesn't magically ward infection, it's an anti-septic.  It's not a vampire/wolfman/X, it's some other coincidental happening.  That kind of thing.  I even wrote out fluff for my players written in the perspective of an NPC present in the world to add verisimilitude (making sure to put enough info to be interesting but not outright explaining things, I think).

But, in the meta-game should I tell them this is all false?  As of the moment they've been told that it's all legit, but what if one of my players decides to pursue a life as a druid or Necrurge?  What if a new player (I only have 3) decides to start as one?  I've already got their first adventure culminating in them assisting an experienced group of demon hunters obliterating a ritual-weaving coven of Demon summoners, which I think will get them into it, but I don't want them to see through it all and think it's just another boring world we're running around in.  I also don't want them to feel as if I'm outright lying to them when / if they find out, either.

M.

split the difference, give them some truth mixed with the perceptions you are already using.

and give seperate players seperate truths, based on their stats and background

They could certainly be members of certain cults/beliefs that are so devoted that they really do believe in a particular superstition from your game even if they denounce pretty much everything else. Alternatively they might know already that it's false and abuse other people's gullibility. Otherwise go with what Vreeg said.