[Yswxomvy]

Some spellcraft, yes is psychological (Oh no! I said it! A'yup), and will effect those who believe, sometimes even those who do not (When someone throws dirt from a graveyard in front of your doorstep right before you step over it, it has a bit of a "what the fuck" factor). HOWEVER, other spellwork will and can effect even the "unbelieving" because it is natural law. I believe the movie The Skeleton Key is a good focus on the psychological effects of magick, if dreadfully boring to sift through. I know that if I cast a spell on someone who does not believe in magick and DOES know I cast it on them, their own disbelief can become their own magick shield (One of the more repeated practices in defensive magick is stating/believing that no harm can come to you). But, if they do not know, they will most likely be effected.
Then again, I've had at least one instance when someone who did not believe me, was effected. They aren't "immune" because it is a real thing that can and does create change, but if they are aware and disbelieving, they can be protected (ironically, their own disbelief in magick acts as magick, but I made that point already, didn't I?).
So basically, yes, it can, no, it can't and it just might.

I remember a tidbit of information once passed to me that Druids and ancient Celtic practitioners used to astral project before a battle to see the enemies ranks and support, etc.
*Just because (supposing) they didn't believe Mr. Druid could astral project, didn't stop him from seeing their cavalry trying to flank his men. Hahahaha.

*The statement above is hypothetical.

Moo.