There are sins we are all prone to struggle with, and even before Christ redeemed us we may have been aware that these things weren't "good" and may have sought to put such behavior behind us. This was done outside of the strength and grace of God and if we did manage to alter our behavior we still hadn't put to death the root of such behavior - our own sinful hearts. More often, though, we were prone to lay aside these things temporarily only to return later - again due to the fact that we hadn't rooted out the cause.
In Christ we have a tool at our disposal that enables us to (should we rely upon it rather than our own will) to dig up and kill the sin that dwells in us - His Spirit. It is only in seeking to lay ourselves down in order to take up Christ that we will truly live. A life lived without the Spirit is a life that will not see victory and freedom from the bondage of the "old man." Paul says in Romans 8 that as many as are led by the Spirit, these are the children God. The Spirit would lead us to mortify our flesh, and so must we if we are to see our lives and the lives of those around us transformed for the glory of God.