skeptic (n.)

also sceptic, 1580s, "member of an ancient Greek school that doubted the possibility of real knowledge," from French sceptique and directly from Latin scepticus "the sect of the Skeptics," from Greek skeptikos (plural Skeptikoi "the Skeptics, followers of Pyrrho"), noun use of adjective meaning "inquiring, reflective." This is related to skeptesthai "to reflect, look, view" (from a metathesized form of PIE root *spek- "to observe"). The name was taken by the disciples of Greek philosopher Pyrrho, who lived c. 360-c. 270 B.C.E.

The extended sense of "one with a doubting attitude, one who suspends judgment and holds that the known facts do not warrant a conclusion" is recorded by 1610s. It is attested by 1630s as "one who doubts or disbelieves the Christian religion," short of absolutely denying it. The sk- spelling is an early 17c. Greek revival and is preferred in U.S.

Skeptic does not mean him who doubts, but him who investigates or researches as opposed to him who asserts and thinks that he has found. [Miguel de Unamuno, "Essays and Soliloquies," 1924]

 The adjective in the sense of "skeptical" is attested from 1570s. As a verb, scepticize (1690s) failed to catch on.