IPA: /ˈbækˌslaɪdə/
KK: /ˈbækˌslaɪdər/
A person who returns to a previous, often worse, state or behavior, especially in a religious context.
After years of dedication, he became a backslider and stopped attending church regularly.
Backslider → It is formed from "back" (from Old English "bæc", meaning the rear or reverse) and "slider" (from the verb "slide", meaning to move smoothly along a surface). The word "backslider" refers to someone who has reverted to a previous, often less virtuous state or behavior, moving backward in their progress.
Think of someone moving 'back' ('back') and 'sliding' ('slider') away from their previous good behavior — that's why a backslider is someone who has regressed.
No commonly confused words.