Many Bible passages emphasize the reality of our security as believers in Jesus Christ: John 10:27-30; 13:1; Romans 8:29-39; Ephesians 1:13; 4:30; Jude 1:24.
But even genuine believers can backslide and lose the joy of their salvation. The New Testament gives many examples of believers who drew back from their fellowship with Jesus Christ: the disciples (Matthew 26:56); Peter (26:69-75); the Christians in Corinth (2 Corinthians 12:20-21); and the Asian churches (Revelation 2:4,14-15,20).
There is a stark difference between backsliding and apostasy—a permanent departure from the faith. A true Christian can backslide, be chastened, and then repent and return (Hebrews 12:6; Revelation 2:5). A person who has merely professed faith without a genuine encounter with Christ may depart, prosper outwardly, and never return. The apostle John said that some who had left the fellowship of believers and were now teaching false doctrine showed by their actions that they never really belonged (1 John 2:19).
The doctrine of eternal security is taught in Scripture, but it is intended to comfort true Christians who are earnestly concerned with living faithfully for Jesus Christ. People who once professed faith and are now living sinfully without remorse should not be comforted by assurances that their profession of faith guarantees their salvation. We gain nothing by examining the nature of the “decision” they made. We need to point out to them that their present lifestyle is out of keeping with their profession by showing them Scriptures such as 1 John 3:4-9. They must be led to self-examination. If they are genuinely saved, God will chasten them (Hebrews 12:6). They will repent and return.
It may be impossible for us to make a judgment as to whether a person is a backsliding Christian or an impostor. Sometimes only time will tell.