Can an athiest be an antichrist?

so the other day my friend and i were arguing about being an antichrist..so i said that anyone who OPPOSES God is one (as in, prefers the devil, etc.). but she said that there is no neutrality. Either your with or not (that would make you against-according to her). So my question is, if lets say you dont believe in him an your an athiest, or agnostic, will that make you an antichrist? OR, if you get involves in satanism and all that, that will make you an antichrist. I appreciate your time in helping me out here :)