1690s, in reference to a specific religious movement, Pietism, from German Pietismus, originally applied in derision to the movement to revive personal piety in the Lutheran Church, begun in Frankfurt c. 1670 by Philipp Jakob Spener (1635-1705). See piety + -ism. With lower-case p- and in reference generally to devotion, godliness of life (as distinguished from mere intellectual orthodoxy), by 1829.
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.