In a nutshell, I would say that as Christians we are no longer slaves to sin. In other words we no longer "desire" to sin because we know that it is offensive to God. So therefore we begin a life of "repentance" where we are constantly aware of our sinful nature and try to and desire to turn away from sin. As we engage in this spiritual warfare, we start to grow and become better at not being so sinful. But it seems that we as Paul might be afflicted by a certain sin or thorn in our side that reminds us who's in control. Anyway, my point is that I believe as long as we desire and fight against our sinful nature we can be assured of our calling.

Y.B.I.C,

Dave

Knowing that a man is not justified by the works of the law, but by the faith of Jesus Christ, even we have believed in Jesus Christ, that we might be justified by the faith of Christ, and not by the works of the law: for by the works of the law shall no flesh be justified. - Galatians 2:16