This is a difficult question, and books could be (and have been) written in response. But let me give you a shorter answer. I’d like to comment on a couple of things before I directly answer your question.
First of all, the Bible clearly teaches that God is the judge, not us. Typically when someone asks this question it is in regards to someone else, not themselves. And, though I’ll give you what I see in Scripture, it is not our role to judge or condemn someone else. God has all the information regarding their heart, their sincerity in receiving Him or not, their sanctification and timeline for that, His plan for their life.
Secondly,how Christians grow in their relationship with God and in their outward lifestyle varies a lot. I was deeply in love with God from the moment I received Him, but it was a couple of years before God straightened out some external behavior. So, to other Christians who didn’t know me, I could “appear” to not be a real Christian. God sanctifies us, changes us, on His time table and it would be a huge disservice to a Christian to bring their eternal security into question over behavior that we judge to be contrary to how a Christian should live.
Regarding losing our salvation: Salvation is offered us purely through Jesus’ death on the cross for us, as a gift of His grace. We, through Christ alone,from the moment we receive Him into our lives, become His child, declared righteous,forgiven, sanctified in His eyes, and have entered a relationship with Him which He intends to be permanent, with the promise of eternal life. This does not hinge on our personal holiness, or levels of faith, or sacrifices, or good works or religious acts. Salvation through Jesus is purely God’s gift, received once by faith. At that moment we have crossed over from death into life, free from the condemnation of the law and placed under His grace, transferred from the kingdom of darkness into the kingdom of light, no longer enemies of God but His child, sealed with the Holy Spirit within us, He has become our God and we His child, and we are at peace with God, reconciled to Him.
The above is based on verses such as:
- Romans 5:8
- Romans 5:1
- Romans 6:22,23
- Romans 10:8-10
- Hebrews 9:15
- Hebrews 8:10-12
- Romans 4:13
- Romans 3:20-24
- Romans 10:4
- John 10:26-28
- Ephesians 1:5-8
- Ephesians 1:13,14
- John 5:24
- 1 John 5:11-13
Some may ask…”what if they refuse to repent of their sins?” Again,we are not the judge. They may be resisting God’s work in their life, resisting the Holy Spirit’s prompting, but I don’t think that would necessarily bring their eternal security into question. God is very patient and allows all of us to resist His prompting in our lives. Some of us change in an area right away, for some it takes longer. If someone becomes a Christian and there is never any change in their heart or lifestyle over time, then it’s possible that person never really wanted Christ in their life to begin with or never understood what becoming a Christian is all about. Romans 12:1,2 talks about how a Christian grows. As a Christian yields to God and spends time in His Word, he or she will be changed by God. In Romans 7, Paul talks about how difficult it is for him to overcome covetousness, and sees his hope as God’s acceptance of him and ability to change him, expressed in Romans 8.
Some may ask…”what if a Christian renounces God?” Here again, let me say that we don’t know if this is a temporary place in their life or not. I clearly remember feeling the weight of responsibility as a Christian, and wondered if I should walk away from it all. For many Christians who don’t understand God’s grace, they are then living under the law, and often feeling like they can never please God.They don’t understand the freedom they have in Christ and how to rest in God’s ability to guide, teach and change them. So some Christians do not follow God because they don’t understand living by faith, the Spirit-filled life,or living under God’s grace. For others who seem to walk away from God because they simply find temptations more appealing, we have Jesus’ parable of the Prodigal Son(Luke 15) and how the father welcomes him back. The son never was disowned by the father, but took himself away from being in relationship with his father and suffered in this life until he went back to his father. I think this is true for Christians who aren’t in fellowship with God. They are likely to be involved in things that are destructive to their lives here on earth. But I don’t think it means they are no longer Christians.