The Collaborative International Dictionary of English v.0.48:

Believer \Be*liev"er\, n.
   1. One who believes; one who is persuaded of the truth or
      reality of some doctrine, person, or thing.
      [1913 Webster]

   2. (Theol.) One who gives credit to the truth of the
      Scriptures, as a revelation from God; a Christian; -- in a
      more restricted sense, one who receives Christ as his
      Savior, and accepts the way of salvation unfolded in the
      gospel.
      [1913 Webster]

            Thou didst open the Kingdom of Heaven to all
            believers.                            --Book of Com.
                                                  Prayer.
      [1913 Webster]

   3. (Eccl. Hist.) One who was admitted to all the rights of
      divine worship and instructed in all the mysteries of the
      Christian religion, in distinction from a catechumen, or
      one yet under instruction.
      [1913 Webster]

WordNet (r) 3.0 (2006):

believer
    n 1: a supporter who accepts something as true [syn: believer,
         truster]
    2: a person who has religious faith [syn: believer,
       worshiper, worshipper]