Devil's Advocate. In the Roman Catholic Church, where the term had its origin, "the devil's advocate" is one appointed to present the arguments against a proposed canonization as a saint. He is, indeed, therefore, as Fowler insists, "the blackener of the good" rather than "the whitewasher of the wicked." None the less the general public, lamentably ignorant of holiness in all its ways and forms, uses the phrase to mean the advocate of a bad cause or one who injures a cause by his advocacy of it, and has so used it so long that these meanings must be accepted as standard.
A Dictionary of Contemporary American Usage, by Bergen Evans and Cornelia Evans, p.133