(n.) One who promotes faction.

Young Turk apostate backslider bolter breakaway collaborationist collaborator convert defector demurrer denominationalist deserter dissenter dissentient dissident divisive factionist fifth columnist heretical heterodox misbeliever mugwump nonconformist objector opinionist opposition voice proselyte protestant protester quisling radical rebellious recidivist recreant recusant renegade renegado renegate reversionist runagate schismatical seceder secessionist sectarian sectary separatist skeptic strikebreaker tergiversant tergiversator traitor turnabout turncoat turntail unconventional unorthodox