So I find it to be a law that when I want to do right, evil lies close at hand. For I delight in the law of God, in my inner being, but I see in my members another law waging war against the law of my mind and making me captive to the law of sin that dwells in my members. (Romans 7:21-23 ESV)
John Stott writes, "...why does Paul describe his experience in terms not only of conflict but of defeat? why does he say not only that he wants to do good, but that he does not and cannot do it? The simple answer, surely, is this. In the previous paragraph (verses 7-13) he has shown that as an unbeliever he could not keep the law. In this paragraph (verses 14ff.) he shows that even as a Christian believer by himself he still cannot keep the law. He can recognize the goodness of the law, he can delight in the law, and he can long to keep the law, none of which was possible to him as an unbeliever. But the flesh, his fallen nature, which was his undoing before his conversion, leading him to sin and death, is still his undoing after his conversion--unless the power of the Holy Spirit subdues it (which is what he comes to later, in chapter 8). Indeed, an honest and humble acknowledgement of the hopeless evil of our flesh, even after the new birth, is the first step to holiness. To speak quite plainly, some of us are not leading holy lives for the simple reason that we have too high an opinion of ourselves. No man ever cries aloud for deliverance who has not seen his own wretchedness. In other words, the only way to arrive at faith in the power of the Holy Spirit is along the road of self-despair. No device exists to settle this issue for good. The power and subtlety of the flesh are such that we dare not relax one moment. The only hope is unremitting vigilance and dependence."
~ John R. W. Stott, Men Made New: an exposition of Romans 5--8 (Inter-Varsity Press, 1966), p. 74.