This is a question that has been debated endlessly over the years. The word “backslider” or “backsliding” does not appear in the New Testament and is used in the Old Testament primarily of Israel. The Jews, though they were God’s chosen people, continually turned their backs on Him and rebelled against His Word (Jeremiah 8:9). That is why they were forced to make sacrifices for sin over and over in order to restore their relationship with the God they had offended. The Christian, however, has availed himself of the perfect, once-and-for-all sacrifice of Christ and needs no further sacrifice for his sin. God himself has obtained our salvation for us (2 Corinthians 5:21) and because we are saved by Him, a true Christian cannot fall away so as not to return.
Christians do sin (1 John 1:8), but the Christian life is not to be identified by a life of sin. Believers are a new creation (2 Corinthians 5:17). We have the Holy Spirit in us producing good fruit (Galatians 5:22-23). A Christian life should be a changed life. Christians are forgiven no matter how many times they sin, but at the same time Christians should live a progressively more holy life as they grow closer to Christ. We should have serious doubts about a person who claims to be a believer yet lives a life that says otherwise. Yes, a true Christian who falls back into sin is still saved, but at the same time a person who lives a life controlled by sin is not truly a Christian.
What about a person who denies Christ? The Bible tells us that if a person denies Christ, he never truly knew Christ to begin with. 1 John 2:19 declares, “They went out from us, but they did not really belong to us. For if they had belonged to us, they would have remained with us; but their going showed that none of them belonged to us.” A person who rejects Christ and turns his back on faith is demonstrating that he never belonged to Christ. Those who belong to Christ remain with Christ. Those who renounce their faith never had it to begin with. 2 Timothy 2:11-13, “Here is a trustworthy saying: If we died with him, we will also live with him; if we endure, we will also reign with him. If we disown him, he will also disown us; if we are faithless, he will remain faithful, for he cannot disown himself.”