"Apostasy" is an English word that means "the renunciation of a religious faith" or "abandonment of a previous loyalty." It was derived from the Greek word "aphistasthai," meaning "to revolt."
"Apostasy" can describe someone who has left a church. A person who commits apostasy is called an "apostate." Religions such as Christianity and Islam view apostasy as a great sin deserving of terrible punishment.
Though it often used in religious context, "apostasy" does not have to refer to religion. An apostate can be anyone who has abandoned a once strongly held belief. A person who was a huge fan of the Detroit Red Wings and abandons this loyalty is an example of apostasy.