"person who initiates into mysteries," 1550s, from Latin mystagogus "a guide to the mysteries," from Greek mystagogos, from mystes "one initiated into the mysteries" (see mystery (n.1)) + agogos "leading, a leader" (see act (n.)). Related: Mystagogic; mystagogical; mystagogy; mystagoguery.
[mystagogue etymology, mystagogue origin, 英语词源]