mid-13c., "image of a deity as an object of (pagan) worship," from Old French idole "idol, graven image, pagan god" (11c.), from Latin idolum "image (mental or physical), form," especially "apparition, ghost," but used in Church Latin for "false god, image of a pagan deity as an object of worship." This is from Greek eidolon "mental image, apparition, phantom," also "material image, statue," in Ecclesiastical Greek," a pagan idol," from eidos "form, shape; likeness, resemblance" (see -oid).
A Greek word for "image," used in Jewish and early Christian writers for "image of a false god," hence also "false god." The Germanic languages tended to form a word for it from the reverse direction, from "god" to "false god," hence "image of a false god" (compare Old English afgod, Danish afgud, Swedish avgud, Old High German abgot, compounds with af-/ab- "away, away from" (source of off) + god).
The older Greek senses sometimes have been used in English. Figurative sense of "something idolized" is first recorded 1560s (in Middle English the figurative sense was "someone who is false or untrustworthy"). Meaning "a person so adored, human object of adoring devotion" is from 1590s.