The Collaborative International Dictionary of English v.0.48:

Worshiper \Wor"ship*er\, n.
   One who worships; one who pays divine honors to any being or
   thing; one who adores. [Written also worshipper.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

worshiper
    n 1: someone who admires too much to recognize faults [syn:
         worshiper, worshipper]
    2: a person who has religious faith [syn: believer,
       worshiper, worshipper]

Moby Thesaurus II by Grady Ward, 1.0:

31 Moby Thesaurus words for "worshiper":
   admirer, adorer, amateur, beadsman, booster, buff, celebrant,
   chapelgoer, churchgoer, collector, communicant, congregation,
   daily communicant, devotee, dilettante, evangelist, fan, fancier,
   follower, groupie, idolater, idolizer, petitioner, prayer,
   revivalist, rooter, suppliant, supplicant, supplicator, venerator,
   votary