Meaning

noun an ardent early supporter of a cause or reform

noun any important early teacher of Christianity or a Christian missionary to a people

noun (New Testament) one of the original 12 disciples chosen by Christ to preach his gospel

originFrom Middle English apostle, from Old French apostle, from Late Latin apostolus, from Ancient Greek ἀπόστολος (apóstolos, “one sent forth, apostle”). Merged with Old English apostol, borrowing from the same Latin source.

Etymology from Wiktionary, CC BY-SA 3.0