Webster's Revised Unabridged Dictionary · 1913

Skeptic

Skeptic , noun

[Greek skeptiko`s thoughtful, reflective, from ske`ptesqai to look carefully or about, to view, consider: compare Latin scepticus, French sceptique. See Scope.]

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.
(Metaphysics) 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.
(Theology) 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

This word and its derivatives are often written with c instead of k in the first syllable, -- sceptic, sceptical, scepticism, etc. Dr. Johnson, struck with the extraordinary irregularity of giving c its hard sound before e, altered the spelling, and his example has been followed by most of the lexicographers who have succeeded him; yet the prevalent practice among English writers and printers is in favor of the other mode. In the United States this practice is reversed, a large and increasing majority of educated persons preferring the orthography which is most in accordance with etymology and analogy.

Skeptic , adjective

[Written also sceptic, sceptical.]

1.
Of or pertaining to a sceptic or skepticism; characterized by skepticism; hesitating to admit the certainly of doctrines or principles; doubting of everything.
2.
(Theology) Doubting or denying the truth of revelation, or the sacred Scriptures.
The skeptical system subverts the whole foundation of morals. — R. Hall

Also: Skeptical