What does the Bible say about sin in a believer's life?

In Romans 7:15-20, the Apostle Paul illustrates the struggle that every believer experiences with sin. He expresses his desire to do good but finds himself repeatedly failing. This struggle is emblematic of the two natures that exist within us: the old fallen nature and the new nature created by God. God permits sin to remain in believers to remind them continuously that salvation is solely by His grace and not by their efforts. This tension between wanting to honor God and the reality of sin within us keeps us humble, relying on God's grace rather than our righteousness.