This difficult doctrine must be distinguished from heresy, which is simply a belief in false doctrine. Apostasy is a deliberate turning away from revealed truth after it has been tentatively received. There are a number of Bible descriptions of it. It is a claim that men know God which is denied by their actions, Tit. 1:16; abiding not in the doctrine of Christ, 2 John 9; a departure from the faith, 1 Tim. 4:1; an unwillingness to endure sound doctrine, 2 Tim. 4:3; a forsaking of the right way, 2 Pet. 2:15; a turning from the holy commandment, 2 Pet. 2:21.
Apostates are not saved persons, because they do not have the Spirit, Jude 19; Rom. 8:9. They are “rocky ground” hearers who receive the Word with joy, Luke 8:13; but afterward fall away, which is literally “apostatize.” The “good ground” hearers receive the Word into their hearts, a much stronger word for receiving it, Mark 4:20, Gr.
Individual apostasy has occurred throughout history. Jude 11 cites three examples from the OT. Judas is the outstanding example in the NT. Corporate apostasy will overtake Christendom in the last days, 2 Thess. 2:3; Rev. 3:14-20. Christ foretold this, Luke 18:8. Jude describes apostates as ungodly men who pervert God’s grace and deny our Lord, Jude 8, 16, 19.
William Evans, The Great Doctrines of the Bible, (Chicago: Bible Institute Colportage Assoc., 1912), WORDsearch CROSS e-book, Under: “Apostasy”.