Old English apostol "messenger," especially the 12 witnesses sent forth by Jesus to preach his Gospel, from Late Latin apostolus, from Greek apostolos "messenger, person sent forth," from apostellein "send away, send forth," from apo- "from" (see apo-) + stellein in its secondary sense of "to send," from PIE *stel-yo-, suffixed form of root *stel- "to put, stand," with derivatives referring to a standing object or place (see stall (n.1)). Cf. epistle.
The current form of the word, predominant since 16c., is influenced by Old French apostle (12c.), from the same Late Latin source. Figurative sense of "chief advocate of a new principle or system" is from 1810. Apostles, short for "The Acts and Epistles of the Apostles," is attested from c.1400.