What does apostasy mean?
The definition of apostasy
A*pos”ta*sy, n.; pl. Apostasies.
Etym: [OE. apostasie, F. apostasie, L. apostasia, fr. Gr. Off and Stand.]

An abandonment of what one has voluntarily professed; a total desertion of departure from one’s faith, principles, or party; esp., the renunciation of a religious faith; as, Julian’s apostasy from Christianity.