Before answering this question, it is important to define a few terms. A "Christian" is someone who has admitted their guilt before God and accepted His free gift of salvation by believing in Jesus as their Savior. As a result, they have been made new (2 Corinthians 5:17), been indwelt by the Holy Spirit (Romans 8:9; 1 Corinthians 12:13; Ephesians 1:13–14), and seek to live a life pleasing to God (Ephesians 4:17–32; Colossians 3:1–17). "Sin" is "missing the mark" of God's perfection. It is anything we do that goes against God. Romans 3:23 says that "all have sinned and fall short of the glory of God." Christians are those who "are justified by his grace as a gift, through the redemption that is in Christ Jesus" (Romans 3:24). However, this does not mean that Christians are immune to sin.
Do Christians sin?
When a person comes to faith in Christ, he is forgiven of all sins, past, present, and future. Before God, that person is deemed to be righteous. However, Christians are also still in the process of being made righteous in practice. This is referred to as sanctification. When a person becomes a Christian, he is freed from the penalty of sin. As he lives his life as a Christian, he is progressively freed from the power of sin (Romans 6:15–23).
Therefore, yes, Christians do sin. First John 1:5–10 says, "This is the message we have heard from him and proclaim to you, that God is light, and in him is no darkness at all. If we say we have fellowship with him while we walk in darkness, we lie and do not practice the truth. But if we walk in the light, as he is in the light, we have fellowship with one another, and the blood of Jesus his Son cleanses us from all sin. If we say we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say we have not sinned, we make him a liar, and his word is not in us." What John is saying is that a follower will not be defined by a lifestyle of sin, but that he will still struggle against sin. However, when a believer sins, God is faithful and just to forgive.
Paul talks about putting off the "old man" and the lifestyle of the fallen world and putting on the "new man" in Christ (Ephesians 4:17–32; Colossians 3:1–17). Christians do not automatically function as fully holy; it is a lifestyle choice of submitting to the Holy Spirit to allow Him to transform us to be more like Christ (2 Corinthians 3:18). In Romans 6 Paul writes, "Let not sin therefore reign in your mortal body, to make you obey its passions. Do not present your members to sin as instruments for unrighteousness, but present yourselves to God as those who have been brought from death to life, and your members to God as instruments for righteousness. For sin will have no dominion over you, since you are not under law but under grace" (Romans 6:12–14). Paul admits his own struggles against sin in Romans 7. In Romans 8 Paul rejoices that there is no condemnation in Christ and nothing can separate us from God's love. Christians do sin, but it is something they struggle against. Christians are not marked by a lifestyle of sin. And even though Christians sin, this does not mean that salvation is at risk or that God stops loving us. All our sin is forgiven when we come to faith in Christ. But when we sin, it can get in the way of our relationship with God. We ask God for forgiveness in order to remove the hindrance from enjoying full fellowship with Him, not in order to be saved again.
Do Christians sin? Yes. Are they marked by a lifestyle of sin? No. Does sin impact a Christian's salvation status before God? No. Can sin hinder the fruitfulness of a Christian's life and his relationship with God? Yes. Is there a remedy for sin? Yes, the remedy is through the abundant and gracious forgiveness of God who treats us as sons, disciplines us for our benefit (Hebrews 12:7–11), and desires that we abide in His love, obey His commands, and experience the fullness of His joy (John 15:9–11).
How is a Christian a new creation? What does 2 Corinthians 5:17 mean?
Are Christians saints or sinners? Or both?
How can I have victory in overcoming sin?
Should Christians confess their sins, even though they are already forgiven?
If Jesus forgave all my sins when I became a Christian, why shouldn't I continue to sin?
Truth about the Christian Life