Apostate

From the Greek apo - histanai 'depart from a stand'.

A person who was once affiliated with a faith group, but has since 'fallen away' and left the group. One group's apostate is generally another group's convert.

Very severe penalties exist in some countries of the world against individuals who abandon the state religion in favour of another faith. It can theoretically mean execution in some Islamic countries. The Roman Catholic church stopped burning European apostates at the stake in 1792.