1540s, "genuinely, with sincerity," Latin, literally "in or with good faith," ablative of bona fides "good faith" (see faith). Originally in English an adverb, later (18c.) also an adjective, "acting or done in good faith." The opposite is mala fide.
"good faith, fair dealing, freedom from intent to deceive," by 1838, English pluralization of bona fide, as though the Latin phrase were a noun. Sense of "guarantees of good faith" is by 1944. The opposite is mala fides "bad faith, intent to deceive."
"one who advocates the contrary side," 1760, translating Latin advocatus diaboli, in the Catholic Church, a promoter of the faith and officer of the Sacred Congregation of Rites whose job it is to urge against the canonization of a candidate for sainthood. "[F]ar from being the whitewasher of the wicked, the [devil's advocate] is the blackener of the good." [Fowler]. Said to have been first employed in connection with the beatification of St. Lorenzo Giustiniani under Leo X (1513-21).