An "apostate" is a person whose gives up a belief, such as a religion, cause or political party. It can be used as a noun or as an adjective. For example, a person who gives up Roman Catholicism is an "apostate Catholic."
The word "apostate" was first used in Middle English sometime between the years 1350 and 1500. It comes from the Latin "apostata," which derives from the Greek "apostenai," meaning "to stand off." Synonyms for "apostate" include nonconformist, defector or traitor. The word has a 10 point value in "Scrabble" and an 11 point value in "Words with Friends."