"belief in more gods than one," 1610s, from French polythéisme (16c.), formed from Greek polytheia "polytheism," polytheos "of or belonging to many gods," from polys "many" (from PIE root *pele- (1) "to fill") + theos "god" (from PIE root *dhes-, forming words for religious concepts).
The various modes of worship, which prevailed in the Roman world, were all considered by the people, as equally true; by the philosopher, as equally false; and by the magistrate, as equally useful. And thus toleration produced not only mutual indulgence, but even religious concord. [Gibbon]
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.
<a href="https://www.etymonline.com/word/polytheist">Etymology of polytheist by etymonline</a>
Harper, D. (n.d.). Etymology of polytheist. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/polytheist