From the American Heritage Dictionary: One who has abandoned one's religious faith, a political party, one's principles, or a cause.
Usually apostates are called ex-members or former members.
Lonnie Kliever, Professor of Religious Studies at the Southern Methodist University, says about apostates:
"There is no denying that these (apostates) present a distorted view of the new religions to the public, the academy, and the courts by virtue of their ready availability and eagerness to testify against their former religious associations and activities."
The full Study: The Reliability of Apostate Testimony About New Religious Movements