Concise Bible Dictionary:
Though the word “apostasy” does not occur in the AV (KJV), the Greek word occurs from which the English word is derived. In Acts 21:21, Paul was told that he was accused of teaching the Jews who were among the Gentiles to apostatize from Moses. Paul taught freedom from the law by the death of the Christ and this would appear to a strict Jew as apostasy. The same word is used in 2 Thessalonians 2:3, where it is taught that the day of the Lord could not come until there came “the apostasy,” or the falling away from Christianity in connection with the manifestation of the man of sin. See ANTICHRIST.
Though the general apostasy there spoken of cannot come till after the saints are taken to heaven, yet there may be, as there has been, individual falling away. See, for instance, Hebrews 3:12; Hebrews 10:26,28, and the epistle of Jude. There are solemn warnings also that show that such apostasy will be more and more general as the close of the present dispensation approaches (1 Timothy 4:1-3). Now a falling away necessarily implies a position which can be fallen from, a profession has been made which has been deliberately given up. This is, as scripture says, like the dog returning to his vomit, and the sow to her wallowing in the mire. It is not a Christian falling into some sin, from which grace can recover him; but a definite relinquishing of Christianity. Scripture holds out no hope in a case of deliberate apostasy, though nothing is too hard for the Lord.
From Anstey’s Doctrinal Definitions:
This refers to the formal abandoning of the faith that a person has professed. Apostasy can be from Judaism (Acts 21:20 – “apostasy from Moses”) or it can be from Christianity (1 Tim. 4:1 – “apostatize from the faith;” 2 Thess. 2:3 – “the apostasy”). It is indicated in Scripture by the expressions “fall away” or “falling away” (Luke 8:13; 2 Thess. 2:3; Heb. 6:6).
Apostatizing is something that only a merely professing believer will do. A real believer may backslide and walk at a distance from the Lord, but he will not abandon the faith. Apostasy is a very solemn thing; once a person apostatizes, there is no hope of him turning around in repentance. Scripture says that it is “impossible” (Heb. 6:4-6). Thus, all such are damned, even though they are still alive in this world! The following passages refer to apostates: Matthew 7:21-23; 12:43-45; 13:5-7, 20-22; Mark 3:28-30; John 15:2, 6; Acts 1:25; Romans 11:22; 1 Cor. 9:27; 10:12; Hebrews 2:1-4; 3:7-15; 6:4-6; 10:26-31; 12:12-29; 2 Peter 2:1, 20-21; Revelation 8:8-12.
We might wonder why the warnings of apostatizing would be given in the Bible—a book which has been written to believers—if believers cannot apostatize. The answer is that in many places the divinely inspired writers of the New Testament were not addressing believers exclusively. Scripture indicates that over time the Christian testimony would deteriorate into an unholy mixture of real and merely professing believers (Matt. 13:25, etc.). This being the case, the writers of the New Testament included in their remarks to the saints warnings for those who were moving among them who were not real. Such people were in danger of apostatizing. These warnings were intended to reach their conscience and awaken them to the reality that they were not saved, and that they needed to be, because if they were to turn their backs on the faith that they had professed, they would be damned forever as apostates!
Many Christians don’t know the difference between backsliding and apostasy and the confusing of these two things has led them to wrong conclusions—one of which is that believers can lose their salvation. The difference between backsliding and apostasy is illustrated in Peter and Judas. They both departed from the Lord; with Peter it was backsliding, and he was restored later; with Judas it was apostasy, and there was no return. W. Scott said, “For backsliding there is a remedy; for apostasy there is none” (Doctrinal Summaries, p. 44). (See Backsliding.)