mid-14c., confourmen, "be obedient (to God), comply," from Old French conformer "conform (to), agree (to), make or be similar, be agreeable" (13c.) and directly from Latin conformare "to fashion, to form, to shape; educate; modify," from assimilated form of com "together" (see con-) + formare "to form" (see form (v.)).
Meaning "to make of the same form or character; bring into harmony, make agreeable," and intransitive sense of "act in accordance with an example" are from late 14c. Sense of "to comply with the usages of the Church of England" is from 1610s; hence conformist (1630s), opposed to non-conformist or dissenter. Related: Conformance; conformed; conforming.
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.