Unbeliever, n.
1. One who does not believe; an incredulous person; a doubter; a skeptic.
2. A disbeliever; especially, one who does not believe that the Bible is a divine revelation, and holds that Christ was neither a divine nor a supernatural person; an infidel; a freethinker.
Syn. -- See Infidel.
Shades of meaning
Fernald has no entry of its own for unbeliever; 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