1650s, "theological doctrine that human will cooperates with divine grace in regeneration" (implying that the fall did not cost the soul all inclination toward holiness), from Modern Latin synergismus, from Greek synergos "working together" (see synergy). Used in non-theological sense "a working together, cooperation" by 1910 (first of medicines).
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.