(n.) A noun suffix denoting an agent, or doer, one who practices, a believer in; as, theorist, one who theorizes; socialist, one who holds to socialism; sensualist, one given to sensuality.

Christian God-fearing man accepter catechumen churchgoer churchite churchman communicant convert cultist daily communicant devotee devotionalist disciple fanatic fideist follower good Christian ist neophyte pietist proselyte receiver religioner religionist religious believer saint the assured the believing the faithful theist true believer truster votary zealot