This is why the secret of sanctification is to develop, within our hearts, a growing intensity of desire to please God, to be obedient to Christ. That’s why we are called to fill our minds with the Word of God, that we may know more of the loveliness of God, the majesty of God, the sweetness and excellence of Christ. The more we know Him, the more we understand how excellent He is. The more we begin to have the mind of Christ, the more we begin to approve the things that God approves and disapprove the things of which He disapproves. Then, our hearts will begin to come into line with our heads.
But, no Christian, in this world, achieves a 100% consistent desire to obey God only. There is a powerful desire left over from the fallen nature. When we have been born again and the Spirit has been shed abroad in our hearts, we have new natures, new desires, new inclinations, new attitudes, new love for the things of God. But, that love is not perfect, it is not pure, it is not yet completely realized in our lives. There is a constant daily struggle and warfare with the old self whose desires are battling the desires of the new self. It is precisely this battle, with which every Christian has struggled, that Paul is setting forth here.
From: The Gospel of God: Romans by R. C. Sproul; reprint (Fearn: Christian Focus, 2011), pp. 155-156. Comment on Romans 7.15. Originally published in 1994.