Now if I do what I do not want, I agree with the law, that it is good. (ESV)
Have you ever been in a situation where you fell into a particular sin that you thought you had gained victory over? It can seem hard to understand how you could fail when the very thing you did repulses you. You know you should not have done it. You have even told others that they should not do it, and yet, here you are, guilty.
Paul says, "If I do what I do not want, I agree with the law, that it is good." What does that mean? First of all, consider that when we fail in our fight over a particular sin, we tend to immediately condemn ourselves. It is natural that we feel shame. That is a byproduct of all sin. But the fact that we can recognize that the sin we did is not what we want to do is actually a positive thing.
Our old sinful self would have just done the sin. Now, since we belong to Jesus Christ, when we do the same sin, we find that we are repulsed by it. We recognize the sin as something that we do not want to do. Why is that? Previously, we were rebels against God's Law. Now, we belong to God. We love the things he loves. Sin cannot be enjoyed like before.
When a true child of God sins, they are immediately faced with the conviction that God brings to them. They know that God's law is good. It is his perfect, holy standard. The very fact that you can acknowledge that the sin you did is not what you want is evidence that you belong to God and you agree that his holy standard is good.
God is changing you. He is daily molding you more and more into his image. Yes, you will sin. But your response to it will prove that you belong to him. And his Holy Spirit will help you in your struggle.