The Collaborative International Dictionary of English v.0.48:

Religionist \Re*li"gion*ist\, n.
   One earnestly devoted or attached to a religion; a religious
   zealot.
   [1913 Webster]

         The chief actors on one side were, and were to be, the
         Puritan religionists.                    --Palfrey.
   [1913 Webster]

         It might be that an Antinomian, a Quaker, or other
         heterodo? religionists, was to be scourged out of the
         town.                                    --Hawthorne.
   [1913 Webster]

WordNet (r) 3.0 (2006):

religionist
    n 1: a person addicted to religion or a religious zealot

Moby Thesaurus II by Grady Ward, 1.0:

56 Moby Thesaurus words for "religionist":
   Brahman, Buddhist, Christian, God-fearing man, Holy Willie,
   Joseph Surface, Mawworm, Pecksniff, Pharisee, Tartuffe, accepter,
   affecter, anthroposophist, believer, canter, canting hypocrite,
   catechumen, churchgoer, churchite, churchman, communicant, convert,
   cultist, daily communicant, devotee, devotionalist, disciple,
   dissembler, dissimulator, fanatic, follower, formalist,
   good Christian, hypocrite, ist, lip server, lip worshiper,
   neophyte, pharisee, pietist, pious fraud, poser, proselyte, ranter,
   receiver, religioner, religious hypocrite, saint, sniveler,
   snuffler, spiritual humbug, theist, truster, votary,
   whited sepulcher, zealot