now its my turn to disagree
a spell is you willing change to happen, therefore it is physcilogical ( I know, I spelled it wrong) however a demon/spirit is an actual being with a mind of its own, therefore even if you don't believe in such things, that doesn't mean it can't harm or possess you
and example- say a guy comes up to you and points a .45 at your head, and pulls the trigger. Now, the everyday person will expect to die, and will. Now say the same thing happens to someone who believes themselves to be impervious to bullets. Will the bullet bounce off his head? will it veer wildly around? no, it won't. Possessions are exactly the same, just because you don't believe in them, doesn't mean you can't be possessed
I might even go so far as to say that if you don't believe in them, you might be even more likely to be possesed by a malicious spirit (remember that actually possessions are very, very rare) because you would have defenses against such a thing.