apostasy

Public rejection of one's faith or taking up of another faith. An example of this is Julian the Apostate, a Roman emperor who rejected his Christian upbringing and the growth of Christianity in the Empire and tried to restore the older religions of Rome and Greece.

Technically, the term is used by the Roman Catholic Church to denote either total defection from the faith, or the abandonment of religious vows by a monk or nun. The Greek word originally meant defection from the military standard but was later applied to a change of religious adherence.