backslider

Etymology

From backslide + -er.

noun

1. A recidivist; one who backslides, especially in a religious sense; an apostate.
   At night the Red Elephant Tusk boomed and groaned among the hills, and the faithful waked and said: ‘The God of Things as They Are matures revenge against the backsliders.’ 1888, Rudyard Kipling, “The Judgement of Dungara”, in Black and White, volume 1, Folio Society, published 2004, page 382
   2009, Andrew F. Cooper, "Confronting Vulnerability through Resilient Diplomacy: Antigua and the WTO Internet Gambling Dispute with the United States" in Andrew F. Cooper and Timothy M. Shaw (eds.), The Diplomacies of Small States: Between Vulnerability and Resilience, Palgrave Macmillan, p. 216, The choice of unilateralism by the US also exposed it to charges that it is a backslider on its WTO commitments.
   You say that you “lapse into coupledom” on occasion. Do you get grief from fellow militant singles for being a backslider? 20 June 2012, Brian Bethune, “Two against one: About coupledom and the stigma of being single”, in Maclean's
   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.