Disbeliever, n.
One who disbelieves, or refuses belief; an unbeliever. Specifically, one who does not believe the Christian religion. I. Watts.
Shades of meaning
Fernald has no entry of its own for disbeliever; he lists it under skeptic.
Where he writes about it:
The skeptic doubts divine revelation; the disbeliever and the unbeliever reject it, the disbeliever with more of intellectual dissent, the unbeliever (in the common acceptation) with indifference or with opposition of heart as well as of intellect. — from skeptic