A recidivist; one who backslides, especially in a religious sense; an apostate.
She married him thinking to change his ways, and for a while he got religion, but he was ever a backslider; she soon began finding bottles stashed about the house.
A disbeliever, a denier: someone who rejects or disbelieves in God as per Islam (Arabic: الله Allāh) or the tenets of Islam; or more broadly any non-Muslim.