"religious song," c. 1000, from Old French ymne and Old English ymen, both from Late Latin hymnus "song of praise," from Greek hymnos "festive song or ode in praise of gods or heroes" (also sometimes of mournful songs), used in Septuagint to translate several Hebrew words meaning "song praising God." Possibly a variant of hymenaios "wedding song," from Hymen, Greek god of marriage, or, as per Watkins, from a PIE root *sam- "to sing" (source also of Hittite išhamai "he sings," Sanskrit saman- "hymn, song"). Evidence for the silent -n- dates from at least 1530.
-ist
word-forming element meaning "one who does or makes," also used to indicate adherence to a certain doctrine or custom, from French -iste and directly from Latin -ista (source also of Spanish, Portuguese, Italian -ista), from Greek agent-noun ending -istes, which is from -is-, ending of the stem of verbs in -izein, + agential suffix -tes.
Variant -ister (as in chorister, barrister) is from Old French -istre, on false analogy of ministre. Variant -ista is from Spanish, popularized in American English 1970s by names of Latin-American revolutionary movements.