The Collaborative International Dictionary of English v.0.48:
Hymnist \Hym"nist\, n.
A writer of hymns.
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