late 12c., "person who speaks for God; one who foretells, inspired preacher," from Old French prophete, profete "prophet, soothsayer" (11c., Modern French prophète) and directly from Latin propheta, from Greek prophētēs (Doric prophatēs) "an interpreter, spokesman, proclaimer; a harbinger" (as cicadas of summer), but especially "one who speaks for a god, inspired preacher or teacher," from pro "before" (from PIE root *per- (1) "forward," hence "in front of, before") + root of phanai "to speak" (from PIE root *bha- (2) "to speak, tell, say").
The Greek word was used in Septuagint for Hebrew nabj "soothsayer, inspired prophet." Early Latin writers translated Greek prophetes with Latin vates, but the Latinized form propheta predominated in post-Classical times, chiefly due to Christian writers, probably because of pagan associations of vates. In English, meaning "prophetic writer of the Old Testament" is from late 14c. Non-religious sense is from 1848; used of Muhammad by 1610s (translating Arabic al-nabiy, and sometimes also al-rasul, properly "the messenger"). The Latin word is glossed in Old English by witga. The Prophets for "the prophetic books of the Old Testament" is by late 14c.