(n.) One who withdraws or separates himself\; especially, one who withdraws from a church to which he has belonged; a seceder from an established church; a dissenter; a nonconformist; a schismatic; a sectary.

abjuratory apostate backslider bolter breakaway collaborationist collaborator convert defector demurrer deserter dissenter dissentient dissident fifth columnist misbeliever mugwump mugwumpian mugwumpish nonconformist objector opinionist opposition voice proselyte protestant protester quisling recidivist recreant recusant renegade renegado renegate renunciative renunciatory repudiative repudiatory reversionist revocatory runagate schismatic seceder secessionist sectarian sectary strikebreaker tergiversant tergiversator traitor turnabout turncoat turntail