On Marriage and Concupiscence

Now this concupiscence, this law of sin which dwells in our members, to which the law of righteousness forbids allegiance, saying in the words of the apostle, “Let not sin, therefore, reign in your mortal body, that you should obey it in the lusts thereof; neither yield your members as instruments of unrighteousness unto sin:” — this concupiscence, I say, which is cleansed only by the sacrament of regeneration, does undoubtedly, by means of natural birth, pass on the bond of sin to a man's posterity, unless they are themselves loosed from it by regeneration.

In the case, however, of the regenerate, concupiscence is not itself sin any longer, whenever they do not consent to it for illicit works, and when the members are not applied by the presiding mind to perpetrate such deeds. So that, if what is enjoined in one passage, “You shall not covet,” is not kept, that at any rate is observed which is commanded in another place, “You shall not go after your concupiscences.” Inasmuch, however, as by a certain manner of speech it is called sin, since it arose from sin, and, when it has the upper hand, produces sin, the guilt of it prevails in the natural man; but this guilt, by Christ's grace through the remission of all sins, is not suffered to prevail in the regenerate man, if he does not yield obedience to it whenever it urges him to the commission of evil.

As arising from sin, it is, I say, called sin, although in the regenerate it is not actually sin; and it has this designation applied to it, just as speech which the tongue produces is itself called “tongue;” and just as the word “hand” is used in the sense of writing, which the hand produces. In the same way concupiscence is called sin, as producing sin when it conquers the will: so to cold and frost the epithet “sluggish” is given; not as arising from, but as productive of, sluggishness; benumbing us, in fact.