The bottom line is this: when someone becomes a Christian, their life is completely changed. To be saved is to be free from the power of the sinful nature. The only reason why the Christian may still struggle is because they have believed the lie that they will always be a sinner, and live accordingly.
How can you be a new creation and still a sinner? These two contradict each other. The new creation cannot be new if he is still old. The old nature is the sinful nature, but the new nature is one of life.
This does not mean that the Christian cannot sin – let me be very clear that this is not what I am saying. What I am saying is that the Christian is free from the power of sin, and must walk in this truth by faith.
To be a Christian is to be included in Christ – to be one with him, your sin covered by his blood and receiving the victory that he won by triumphing over the grave.
Being in Christ, the Christian will look like Christ in thought and conduct. The Lord Jesus uses this illustration: “No good tree bears bad fruit, nor does a bad tree bear good fruit. Each tree is recognized by its own fruit. People do not pick figs from thornbushes, or grapes from briers. The good man brings good things out of the good stored up in his heart, and the evil man brings evil things out of the evil stored up in his heart. For out of the overflow of his heart his mouth speaks” (Luke 6:43-45). In other words, a Christian looks like a Christian. Just like an apple tree will always grow apples and not pears, so a Christian will look like Christ, and not the deceiver, Satan, being held under his influence and living in sin.
Deitrich Bonhoeffer said, “Christianity without discipleship is always Christianity without Christ.” A disciple is a follower of Christ, and discipleship is the act of following Christ. These things we have been discussing is what it is to follow Christ. Without following Christ and being like him, one cannot be a Christian.