Paul shows here that those who are under the law, because they live according to the flesh, are under sin and condemnation. But those who are in Christ are not under condemnation because they do not walk according to the flesh. .
Notwithstanding this law of sin opposing the law of the spirit, and endeavouring to captivate us to sin, we can, by the succour of the grace of Jesus Christ, preserve ourselves from sin and damnation, by resisting sin, and attaching ourselves strongly to Jesus Christ, by faith and charity. Concupiscence is not sin. And they who have received a spiritual birth in Jesus Christ by baptism, and have remained faithful to the grace of their justification, who walk not according to the flesh, but according to the spirit, shall receive a recompense proportionate to their combats and labours. (Calmet)
Paul met the difficulty posed by postbaptismal sin by saying that it is due to our laziness. For now that we are in Christ Jesus we have the power to avoid walking after the flesh, but before that it was a difficult task.
After showing that we are freed from sin [n. 406] and the Law [n. 518] through Christ’s grace, the Apostle now shows that through the same grace we are freed from damnation. First he shows that through the grace of Christ we are freed from the damnation of guilt; secondly, from the damnation of punishment, there [v.10; n. 628] at And if Christ. In regard to the first he does two things. 296 First, he sets forth his intention; secondly, he proves his proposition, there [v.2; n. 600] at For the law of the spirit of life. 596. In regard to the first he does two things: first, he mentions the benefit which grace confers, drawing his conclusion from the foregoing in this way: The grace of God through Jesus Christ has freed me from the body of this death and in this consists our redemption. Therefore, now that we have been freed through grace, there is no damnation left, because the damnation has been removed both as regards guilt and as regards punishment: "It is he himself who grants peace...