backslider
/ˈbækˌslaɪdər/
noun
- A person who returns to a previous, usually worse, behavior or habit, especially in religion or moral conduct.
- The church welcomed the backslider back into the community with open arms.
- He was a former health nut but became a backslider, eating junk food every day.
- The program helps backsliders stay on track with their recovery goals.