So, you are a Christian. You've been following Christ for a couple months or maybe seventy years. You know that God has given his Holy Spirit to you to guide you in life, to remind you of all that Christ taught, and to convict you of sin and righteousness. You know that God is completing His work in you. You know that people are known by their works, whether good or bad. However, there is a sin that you just can't shake. Maybe, there are twenty sins that you just can't shake. Does this mean that you are not actually a Christian? Should a follower of Christ be sinless? How can we reconcile sinful humanity with the work of God?
I'm glad you asked.
The Character of God
First, we need to start with who God is. God is God. As God, He is good. He is righteous. He is holy. There is no sin or evilness in Him. There is no one else that comes close to His goodness. Our human minds cannot fully comprehend his complete holiness, because we are surrounded by a life of brokenness and sin. From the beginning of the Bible, to the end, His perfect character is attested to (Ex 15:11; Rev 15:4).
As a perfect God, He cannot have sin close to Him. This isn't because sin weakens God, but because sin is not able to come near God. When Adam and Eve ate the fruit in the garden, their relationship with God was instantly severed. They died spiritually, because they had introduced sin into the world. God, based upon His justice and holiness, separated humanity from himself for eternity. (Gen 3; Rev 20:11-15)
The Work of Christ
God loved His creation, though, and wanted to reconcile them to Himself. In the fullness of time, He sent His Son to die, taking humanity's punishment on Himself. He also defeated the works of the devil, and took sin away (Rom 5:1-11; 1 Jn 3:5,8). All who place their faith in Christ will be restored back into relationship with God, because of the work of Jesus Christ. That relationship starts at the moment of salvation, and continues throughout eternity.
All who have placed their faith in Christ have been made new (1 Cor 5:17). Their old sinful nature is gone. They now have the ability to live a life of good (Rom 6:18-23). This is because of the power of Christ and the work of the Holy Spirit. In fact, those who are following Christ should understand the seriousness of sin and the holiness of God to the extent that any action against His character would be shunned. Because of what Christ has done, I should not want to sin.
The Reality of Humanity
However, we as humans, even if we are following Christ closely, still have a tendency to sin. Paul speaks of this in Romans 7 when he bemoans his own nature that does what he hates (Rom 7:15). In 2 Corinthians, he talks about a "thorn in the flesh" that tormented him and wouldn't leave (2 Cor 12:7). I believe this is a sin that Paul struggled with his whole life. We are broken creatures. Even though we have the ability to do good, and even though we are not slaves of sin anymore, we still choose to sin. We still choose to grieve our God and our Savior.
We will be broken until Christ comes again and ushers us into His eternal kingdom. At this point, he will give us glorified bodies like His (1 Jn 3:2; Phil 3:20-21). We will be made complete, and we won't have to struggle with sin anymore. That will be a glorious day.
God understands our nature. He knows that we are broken and have a propensity for sin. That is why He sent Christ to die. Christ's death paid the penalty for all our sin: what we committed in the past, present, and future (1 Jn 2:1-2). When God looks at us, even though we don't meet His standard, He sees the righteousness of His Son on us. This grace gives us hope and endurance for this life.
This grace should also cause us to think twice before sinning against the character of our God and our Savior. For, if we truly understood the seriousness of sin, the character of our God, and amazingness of His grace, we would be running in the opposite direction of sin.