Why is the ninth month called September?
late 14c., "renunciation, abandonment or neglect of established religion," from Latin apostasia, from later Greek apostasia, from apostasis "revolt, defection," literally "a standing off" (see apostate). General (non-religious) sense is attested from 1570s.
the total rejection of Christianity by a baptized person who, having at one time professed the Christian faith, publicly rejects it. It is distinguished from heresy, which is limited to the rejection of one or more Christian doctrines by one who maintains an overall adherence to Jesus Christ.