A falling away. The common classical use of the word is, a political defection (Gen 14:4, Sept.; II Chron 13:6, Sept; Acts 5:37). In the New Testament its more usual meaning is a religious defection (Acts 21:21; I Tim 4:1; Heb 3:12). This is called "apostasy from the faith" (apostasia a fide): a secession from the Church, and a disowning of the name of Christ. Some of its peculiar characteristics are mentioned: seducing spirits, doctrines of demons, hypocritical lying, a seared conscience, forbidding of marriage and of meats, a form of godliness without the power (I Tim 4:1; II Tim 3:5).
The grave nature of apostasy is shown by such passages as Hebrews 10:26-29; II Peter 2:15-21; John 15:22. Apostasy as the act of a professed Christian, who knowingly and deliberately rejects revealed truth regarding the deity of Christ (John 4:1-3) and redemption through his atoning sacrifice (Phil 3:18; II Peter 2:1), is different from error, which may be the result of ignorance (Acts 19:1-6), or heresy which may be the result of falling into the snare of Satan (II Tim 2:25, 26). Both error and heresy may accordingly be consistent with true faith, but not from the outward profession of it (II Tim. 3:5).
Apostasy whether among the angels, in Israel or in the Church is irremediable, and awaits judgment. Mankind's apostasy in Adam (Gen 3:6,7) is curable only throught the sacrifice of Christ.
Ungers Bible Dictionary