The problem here is he's trying to use Clean QuakeC (For Quake) with Kurok (the mod). If he wants to use Kurok (the mod) as a base, he should bug MDave for a Clean QuakeC (For Kurok the mod).
Alternately, he could just:
1. Get a replacement model for progs/invulner.mdl. For the sake of example, perhaps this if he doesn't mind a certain blue pill: (Just put in your mod's progs folder and rename it invulner.mdl)
2. Get a modified and or edit the gfx.wad file with either fimg or adquedit to replace the pentagram powerup icon(s).
3. Modify the quakec and engine to remove references to pentagrams and 666.
4. Get a third party base theme map pack (or collection of individual maps) to play with until you learn to make your own maps. If memory serves, this one here should suffice for this, and probably lacks pentagrams if I remember right. (it's been a while since I played this one mind you)
If you still have a problem because of the monsters, I suggest you pretend that they are actually just mutant creatures (like the doom movie), not demonic hellspawn.
To be quite honest though, I see nothing terribly satanic about the stock quake monsters. Fiend you say? Pretend it's a Pigboar-Goat hybrid, Shambler? Mutant gorilla / yeti. Death Knight? Angry Viking, etc.
Or you could go check out some other mods and loot them for models with different skins.
Honestly though, if you want to learn to mod for the quake engine, this would be a good place for you to start learning how to actually mod for it. (replacing / adding content is after all what modding is about)