late 14c., omelye, from Old French omelie "homily" (12c., Modern French homélie), from Church Latin homilia "a homily, sermon," from Greek homilia "conversation, discourse," used in New Testament Greek for "sermon," from homilos "an assembled crowd," from homou "together" (from PIE *somalo-, suffixed form of root *sem- (1) "one; as one, together with") + ile "troop, band, crowd" (cognate with Sanskrit melah "assembly," Latin miles "soldier"). Latinate form restored in English 16c. A collection of them is a homiliary (1844).
-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.