Freethinker, n.
One who speculates or forms opinions independently of the authority of others; esp., in the sphere or religion, one who forms opinions independently of the authority of revelation or of the church; an unbeliever; -- a term assumed by deists and skeptics in the eighteenth century. Atheist is an old-fashioned word: I'm a freethinker, child. Addison.
Syn. -- Infidel; skeptic; unbeliever. See Infidel.
Shades of meaning
Fernald has no entry of its own for freethinker; he lists it under skeptic.
He lists it without comment; the entries above explain its neighbors.