Apostasy

Apostasy (Greek απο, apo, "away, apart", στασις, stasis, "standing") is the renunciation of one's religion. It is customarily considered a vice, a corruption of the virtue of piety in the sense that when piety fails, apostasy is the result. However, most converts[?] to a new religion can also be considered apostates from a previous belief. The word is also used to refer to renunciation of belief in a cause other than religion.

One who is "guilty" of apostasy is a apostate, or one who apostatizes.

See Virtue and Great Apostasy.