(n.) One who backslides.

Sabbath-breaker atheist atheistic backslider backsliding blasphemer blasphemous bolter collaborationist collaborative collaborator convert defector degenerate demurrer deserter disloyal dissenter dissentient dissident faithless fallen fallen from grace fifth columnist impious irreligious irreverent lapsed mugwump nonconformist objector opinionist opposition voice profanatory profane proselyte protestant protester quisling rat recidivist recidivistic recreant recusant renegade renegado renegate reversionist runagate sacrilegious sacrilegist schismatic seceder secessionist sectarian sectary separatist strikebreaker tergiversant tergiversating tergiversator traitor traitorous treasonable treasonous turnabout turncoat turntail unbeliever undutiful