As John Newton wisely wrote in his popular hymn "Amazing Grace," we all were lost, but if we've given our lives to Christ, we've been found and saved from sin. We'd like to think of our post-conversion lives as a spotless and continuous pursuit of growing into the likeness of Christ. But we can persist in sin for years without changing.
Paul taught that, while we may move away from obvious and evident sin, we contend with inner attitudes that cause us to move away from God's grace. Or we persist in sinful pursuits and justify them—as John Newton did when he continued to work in the slave trade after his conversion. Dealing with our sinful nature is a process over time, as made clear in Romans 7:7-25 and 1 John 1:8-2:6. Sin constantly seeks to draw us away from God, and we regularly need to repent: 1 John 3:4-10 and Luke 15:11-32.
It is one thing to receive the salvation we have in Jesus and to be delivered from our sins. It is quite another to fully receive Christ in our lives and to be Christ in the world. We have been reconciled, but we struggle to incorporate fully the breadth and depth of that reconciliation and to live it out. That's why repentance is a necessary and ongoing Christian discipline. We strive to be sinless, but we do fall into sin.
As we move toward spiritual maturity, we become more aware of our wretchedness in contrast to God's glory—and more aware of God's amazing grace (Ephesians 4:17-25). It is a constant work of grace God does in our lives, if we continue to confess and move toward his holiness.