If u belief, then it exist
If u dont, then it doesnt

It depends on your religious beliefs.
What does your religion tells you about sorcery,magic,illusions etc,etc.

In some religions, theirs is a belief that it exists, that evil exists.
So to ward of evil magic you have to use good magic.
Satan exists as some religions believe that it does,
God has allowed him to use his influence on the weak one that doe not have positive belief in God.

So the chance of one getting hit by satans magic is strong, because that person mentals state has allowed him to do so.