Old English apostol "messenger," especially the twelve witnesses sent forth by Jesus to preach his Gospel (Luke vi.13), from Late Latin apostolus, from Greek apostolos "messenger, envoy," literally "person sent forth," from apostellein "send away, send forth," from apo "off, away from" (see apo-) + stellein in its secondary sense of "to send," from PIE *stel-yo-, suffixed form of root *stel- "to put, stand, put in order," with derivatives referring to a standing object or place. Compare epistle.
The current form of the word, predominant since 16c., is influenced by Old French apostle (12c., Modern French apôtre), from the same Late Latin source. Meaning "missionary who brings Christianity to a new region or people" is from early 15c. Figurative sense of "chief advocate of a new principle or system" is from 1810. The New Testament book title Apostles (c. 1400) is short for "The Acts and Epistles of the Apostles."