A renegade is a person who abandons a faith or cause, or, commonly, someone who is rebellious or an outlaw.
Renegade comes to English from the Spanish word renegado, which itself was derived from the medieval Latin renegatus, the root of which also gave us the term "renege", which means to back out of an agreement. In Spanish the term originally denoted a person who renounced Christianity and converted to Islam. Currently the term also refers to someone who abandons one cause for another. Supporters of the cause which the "renegade" abandoned may call that person a traitor.  A person who converts from atheism to Christianity would not usually be called a renegade.