Source : Webster's Revised Unabridged Dictionary (1913)

Apostle \A*pos"tle\, n. [OE. apostle, apostel, postle, AS.
   apostol, L. apostolus, fr. Gr. ? messenger, one sent forth or
   away, fr. ? to send off or away; ? from + ? to send; akin to
   G. stellen to set, E. stall: cf. F. ap[^o]tre, Of. apostre,
   apostle, apostele, apostole.]
   1. Literally: One sent forth; a messenger. Specifically: One
      of the twelve disciples of Christ, specially chosen as his
      companions and witnesses, and sent forth to preach the
      gospel.

            He called unto him his disciples, and of them he
            chose twelve, whom also he named apostles. --Luke
                                                  vi. 13.

   Note: The title of apostle is also applied to others, who,
         though not of the number of the Twelve, yet were equal
         with them in office and dignity; as, ``Paul, called to
         be an apostle of Jesus Christ.'' --1 Cor. i. 1. In
         --Heb. iii. 1, the name is given to Christ himself, as
         having been sent from heaven to publish the gospel. In
         the primitive church, other ministers were called
         apostles --(Rom. xvi. 7).

   2. The missionary who first plants the Christian faith in any
      part of the world; also, one who initiates any great moral
      reform, or first advocates any important belief; one who
      has extraordinary success as a missionary or reformer; as,
      Dionysius of Corinth is called the apostle of France, John
      Eliot the apostle to the Indians, Theobald Mathew the
      apostle of temperance.

   3. (Civ. & Admiralty Law) A brief letter dimissory sent by a
      court appealed from to the superior court, stating the
      case, etc.; a paper sent up on appeals in the admiralty
      courts. --Wharton. Burrill.

   {Apostles' creed}, a creed of unknown origin, which was
      formerly ascribed to the apostles. It certainly dates back
      to the beginning of the sixth century, and some assert
      that it can be found in the writings of Ambrose in the
      fourth century.

   {Apostle spoon} (Antiq.), a spoon of silver, with the handle
      terminating in the figure of an apostle. One or more were
      offered by sponsors at baptism as a present to the
      godchild. --B. Jonson.

Source : WordNet®

apostle
     n 1: an ardent early supporter of a cause or reform; "an apostle
          of revolution"
     2: any important early teacher of Christianity or a Christian
        missionary to a people
     3: (New Testament) one of the original 12 disciples chosen by
        Christ to preach his gospel