A form of this question came up last Sunday, so thought I’d put up some thoughts on the topic. Sorry, it’s a little longer than our normal posts:
The Bible teaches that if someone claiming to be a Christian goes astray and doesn’t turn back to God in repentance, it shows they never had salvation in the first place. We need to hold two things together here:
1) One is the idea of the perseverance of the saints – if God has called us (ie. we believe in Jesus), then he will see us persevere in that belief till the end (Rom 8:30 says this clear enough – those he predestined, he also called; those he called, he also justified; those he justified, he also glorified). Our salvation is all about Jesus start to finish and so the focus is not on our faith or strength of our faith but on Jesus and his finished work on our behalf – he lived the perfect life for us.
2) However, we need to not be presumptuous and think that we can ‘believe in Jesus’ and live however we want. The life of faith has a shape. We need to take seriously the warnings of scripture. A passage like Hebrews 10:23-27 offers a stern warning in the context of Christians who have given up meeting together. If we keep on sinning after receiving a knowledge of the truth, we can expect to face judgment as an enemy of God.
The child of God, as they hear the warnings of God, they believe God’s Word, they repent from their sin, trust God for forgiveness through Jesus and continue walking with him. For one who is not his child, they ignore the warnings of God, whether or not they claim to ‘believe in Jesus’, their actions betray that they do not actually believe God’s Word, they have shown practically that they are not trusting in Jesus, though formally they still claim to. Salvation is a gift through Christ, but a person refusing to heed the warnings of the Scripture, is refusing God’s gift.
The truth of the two points above is the reason Jesus institutes church discipline (Matthew 18:15-18). We accept one another as brothers and sisters in Christ based on a confession of faith. However, as we see sin in one anothers lives, we point that out and if there is repeatedly no repentance, we treat that person as though they are no longer a brother or sister, as a warning, that they might get a clear message that they need repent or they will not be found in heaven, regardless of their confession.
Practically, in in research that has been done in an Australian context, not many people decide overnight to give up on Christ. They fall into sin (things like unforgiveness of someone who’s hurt them etc.), they slowly drift out of church, and after 5-7 years of not going to church and being part of a church family, though they may still call themselves Christians, they believe a whole different Jesus from the Bible. Jesus may be seen as Saviour, but certainly not as Lord, which is the confession under which the apostle Paul says we are saved (Romans 10:9).
So in summary, the person who loses their way, unless they repent before they die, are showing that never actually had salvation.
(On a related topic – repentance – have a read of this wonderfully heart warming article by Tim Keller on All of Life as Repentance)