Thomas Aquinas, on Rom 6:6

“Knowing this, that our old man is crucified with him, that the body of sin may be destroyed, to the end that we may serve sin no longer.”

“After showing that we should not continue to sin after baptism has made us dead to sin, the Apostle now shows that we have the means to do this. Concerning this he does two things. First, he indicates the benefit we have received; second, the effect of this benefit, at for he that is dead. Concerning the first he does two things. First he sets out the benefit we obtain; Second, the effect of this benefit, at that the body of sin. First, therefore: we have said that the baptized should walk in newness of life, namely, by desisting from sin. To anyone who says that this is impossible he responds, knowing this, that our old man, i.e., the oldness caused by sin, is crucified, i.e., put to death by the cross of Christ, with him, namely, with Christ. For as was stated above, man's oldness was brought about by sin—in the sense that the goodness of his nature was corrupted by sin—and is predominant as long as he continues in sin. But because what is predominant in a man characterizes the man himself, it follows that the oldness of sin in a man subject to sin is an apt description of the man himself. That is why he says the old self. Now the oldness of sin can refer to the guilt of sin or to the stain of actual sins or even to the habit of sinning, which engenders a sort of compulsion to sin, or even to the fomes, or concupiscence, deriving from the sin of the first parent. Thus, therefore, our old self is said to be crucified together with Christ, inasmuch as the aforesaid oldness is removed by the power of Christ; either because it has been entirely removed, as the guilt and stain of sin are entirely removed in baptism, or because its force has been diminished, i.e., the force of the fomes or even of the custom of sinning: having canceled the bond which stood against us with its legal demands; this he set aside, nailing it to the cross (Col 2:14). Then when he says, that the body of sin, he mentions the twofold effect of this benefit, the first of which is the removal of previous sins. Hence he says: that the body of sin may be destroyed. The body of sin is the assemblage of evil deeds, just as the assemblage of members makes one natural body: his body is like molten shields (Job 41:6). The second effect is that it makes us beware of future sins; hence, he adds: to the end that we may serve sin no longer. For a man is a slave of sin, when he obeys the concupiscence of sin by consenting to and doing the sin: everyone who commits sin is a slave to sin (John 8:34).”

Imported from an open dataset — not yet checked against the printed edition.