The Collaborative International Dictionary of English v.0.48:

Hymnist \Hym"nist\, n.
   A writer of hymns.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

25 Moby Thesaurus words for "hymnist":
   arranger, ballad maker, balladeer, balladist, balladmonger,
   composer, contrapuntist, ethnomusicologist, harmonist, harmonizer,
   hymnographer, hymnologist, librettist, madrigalist, melodist,
   melodizer, musicographer, musicologist, orchestrator, scorer,
   song writer, songsmith, symphonist, tone poet, tunesmith