magic as we know is the manipulation of energy . Now if that energy is aimed at you and you are not protected from it , it will have an effect on you . If the person sending the curse to you is more powerful then you and can overpower your protection then something will happen to you . Just because someone does not believe in something does not mean you can't feel the effects of that energy . No one is all powerful no matter what you may believe you are .
You must remember that if someone is protected and another person ( the curser ) knows this they can still harm you by harming those you care about . Would then that curse still not harm you ...?.