early 15c., "creed, summary, religious belief," from Late Latin symbolum "creed, token, mark," from Greek symbolon "token, watchword, sign by which one infers; ticket, a permit, licence" (the word was applied c. 250 by Cyprian of Carthage to the Apostles' Creed, on the notion of the "mark" that distinguishes Christians from pagans), literally "that which is thrown or cast together," from assimilated form of syn- "together" (see syn-) + bole "a throwing, a casting, the stroke of a missile, bolt, beam," from bol-, nominative stem of ballein "to throw" (from PIE root *gwele- "to throw, reach").
The sense evolution in Greek is from "throwing things together" to "contrasting" to "comparing" to "token used in comparisons to determine if something is genuine." Hence, "outward sign" of something. The meaning "something which stands for something else" first recorded 1590 (in "Faerie Queene"). As a written character, 1610s.