Ongoing struggle with a particular sin or sins may cause us to question whether we are truly Christians or not. This is an important question. A Christian is someone who is repenting (turning from their sinful rebellion and self-reliance) and believing / trusting the gospel of Jesus (Mark 1:15, Acts 20:21). To become a Christian is to be made a new creation (2 Corinthians 5:17) or be born again (John 3:3-8, 1 Peter 1:23), with new desires: loving God, wanting to live for him and please him. Yet, Christians will still battle with ‘indwelling sin’, there will be a conflict between the Spirit and the sinful nature (Galatians 5:17).
The children of God are free through regeneration from bondage to sin. Yet…their still remains in them a continuing occasion for struggle…a smoldering cinder of evil, from which desires continually leap forth to allure and spur you to commit sin. (John Calvin, Institutes, 3.3.10)
Through the gospel and the Spirit Christians have real power for ongoing growth in this battle (Romans 6:11-14, Titus 2:11-13). God tells us that If we claim to be without sin, we deceive ourselves (1 John 1:8), yet warns us that No one who is born of God will continue to sin, because God’s seed remains in him; he cannot go on sinning, because he has been born of God (1 John 3:9). If we continue to wilfully give in to a particular sin, it may be because we do not truly know God, or it may be that we have not grasped the power of the gospel and the Spirit in overcoming sin.
The Bible calls us to examine ourselves and test ourselves to see whether we are in the faith (2 Corinthians 13:5), yet there is a danger in becoming overly introspective and looking at our own actions more than the finished work of Christ. God calls us to repent and believe the gospel. Believe the gospel: trust that Jesus death on the cross is sufficient for God to forgive you of sin you are presently conscious of and all other sin too. Repent: Acknowledge the seriousness of your sin against God and your inability to save yourself, seek God’s power through the gospel, the Spirit, and the help of other Christians to make practical changes in relation to specific sins. The gospel gives us power for real change. A Christian does not have to remain enslaved by a particular sin. Legalism says, “You must not . . .” The gospel says, “You need not . . . because God is bigger and better than sin.” (Tim Chester)
In this clip David Powlison answers someone who is wondering whether they could truly be a Christian if they have been involved in ongoing secret sin:
The article David refers to is: Making All Things New: Restoring Joy to the Sexually Broken. Video and audio of his talk on this topic here .
Other resources: How Can I be Sure I’m a Christian? by Donald Whitney. You Can Change: God’s transforming Power for our Sinful Behaviour and Negative Emotions by Tim Chester.