The sins that you fight in your fight for joy — the joylessness that you fight — is a forgiven joylessness. The only sin that you can defeat is a defeated sin. The only sin which is prompted by finding your pleasure somewhere other than in God that you can defeat — the only way to defeat it — is to see it as a punished sin, a covered sin.
This is the mystery of the Christian life: fighting as a justified sinner. My sins — all of them — were covered by Jesus. Therefore, when I make war on them, I know they are already defeated, covered, punished. And those are the only ones I can get any victory over. If I turn this around and begin to think, “Now there are some sins and I am going to attack them and defeat them so that God will accept me,” you are dead — dead in the water.
There is only hope if we get the order of justification and sanctification right and do not mingle the two, which is the great Roman Catholic error: the mingling of sanctification and justification. Getting right with God on the basis of Christ alone by grace alone through faith alone to the glory of God alone learned about from the Bible alone is biblical truth. And then, when you are standing right with God by faith alone, you make war on sin. That is the fruit and evidence that you are right with God. It is not the way you get right with God. It is the fruit and evidence.
A person who is cavalier about their sin — who sees no problem with living in sin because they are saved by grace — isn’t saved. At least, we have no biblical warrant for saying they are. There may be a season of backsliding, so we mustn’t judge too quickly, but a person who goes on treating the sin in their own life in a cavalier, nonchalant way, using grace as license, has not understood what it is to be born from above, because a new person comes into being.
Source: Desiring God | John Piper