Board Thread:Warhammer 40k General Discussion/@comment-6134883-20130417202949/@comment-3258762-20130617162223

All supernatural things come from the Warp. The Warp is raw emotion. If you believe you will become stronger because the Emperor watches over you, it will reflect in the Warp, and if the veil  between the Materium and the Warp is thin enough, it will reflect in you by making you stronger. For example.

However, to do it requires incredible, fanatical belief, either in some god or in yourself. Like the Space Wolves, their Rune Priests (Librarians) are protected from Daemonic Possession only because they believe they are immune to it (Because they don't see themselves as psykers), thus giving them the same protection aLibrarian who believes his training will protect him has.