An apostate is one who once had a profession of Christianity and partook of its blessings, but then fell away from “the faith”, never truly having possessed “saving faith”.
Merely rejecting the gospel does not constitute apostasy. Hebrews 6 speaks of those who “were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, and have tasted the good word of God, and the powers of the world to come” and yet remained unconverted. If they were to reject that testimony of the Spirit, “it is impossible… to renew them again unto repentance.” Incorrect use has been made of this verse to teach that the believer’s security is conditional, and that it is possible, once saved, to be lost again. This is false, for it denies many other scriptures, such as John 10:27-29. Only an unbeliever can apostatize, because they never had saving faith to begin with.
Apostasy and Apathy. A great deal of confusion comes in if we don’t understand what an apostate is, and what a backslider is. A backslider is one with real faith but who gets off the path and goes into sin. Their eternal destiny is secure, but their life may end up being a loss for Christ. Hebrews, 2 Peter, and Jude take up the subject of apostasy at length. But, what if I have real faith? Is there nothing in those scriptures for my heart and conscience? Of course. Along with actual apostasy is the spirit of apostasy, or apathy. We are warned that the same spirit and principle that actual apostates operate on can infect those who are true believers.
Children of the Devil. Apostates in John’s Gospel are called “children of the Devil” (John 8:37-47). It is very important to see that not every unsaved person is a child of the Devil. All are children of Adam, in bondage to sin, children of disobedience, and children of wrath (Eph. 2:3), meaning that we are all born sinners, and heading down the road that leads to destruction. But a child of the devil is an apostate who has hardened himself against the truth, and thus given himself up to the service of Satan. In scripture we only find the Lord Himself identifying those who are the Devil’s children, or those with apostolic authority, like Paul in Acts 13:10.