I believe that we have to put on the Lord Jesus Christ for this to be true. It is His righteousness that I put on. If I have not done that then my own righteousness is as filthy rags. I believe that the Romans 7 guy was a Christian so I disagree with what he said about that guy not being a Christian. Paul is talking about his own struggle with sin. He won his battle through the Lord Jesus as he continues to explain in chapter 8. When we first become a Christian we need to grow, but our immaturity does not make us not a Christian. As we mature in Christ we sin less and I believe we can get to the point where we do not sin. Some people may never grow up in Christ and may always feed on the milk of the word instead of the meat of the word. I believe they are still Christians. I believe many of us go through struggles with sin as described in Romans 7. Repentance in many cases is a daily battle of saying no to our flesh and sin and saying yes to the way of Jesus. I think the more we are filled with His Holy Spirit the easier the battle is. The more of Christ that we put on the easier the battle becomes. His yoke is easy and His burden is light.