skeptic

skeptic is a Noun

Definition:

[1] One who is yet undecided as to what is true; one who is looking or inquiring for what is true; an inquirer after facts or reasons.

[2] A doubter as to whether any fact or truth can be certainly known; a universal doubter; a Pyrrhonist; hence, in modern usage, occasionally, a person who questions whether any truth or fact can be established on philosophical grounds; sometimes, a critical inquirer, in opposition to a dogmatist. All this criticism [of Hume] proceeds upon the erroneous hypothesis that he was a dogmatist. He was a skeptic; that is, he accepted the principles asserted by the prevailing dogmatism: and only showed that such and such conclusions were, on these principles, inevitable. Sir W. Hamilton.

[3] A person who doubts the existence and perfections of God, or the truth of revelation; one who disbelieves the divine origin of the Christian religion. Suffer not your faith to be shaken by the sophistries of skeptics. S. Clarke.