Augustine of Hippo, On Marriage and Concupiscence I.28

Chapter 28 [XXV.]—Concupiscence Remains After Baptism, Just as Languor Does After Recovery from Disease; Concupiscence is Diminished in Persons of Advancing Years, and Increased in the Incontinent.

If the question arises, how this concupiscence of the flesh remains in the regenerate, in whose case has been effected a remission of all sins whatever; seeing that human semination takes place by its means, even when the carnal offspring of even a baptized parent is born: or, at all events, if it may be in the case of a baptized parent concupiscence and not be sin, why should this same concupiscence be sin in the offspring?—the answer to be given is this: Carnal concupiscence is remitted, indeed, in baptism; not so that it is put out of existence, but so that it is not to be imputed for sin. Although its guilt is now taken away, it still remains until our entire infirmity be healed by the advancing renewal of our inner man, day by day, when at last our outward man shall be clothed with incorruption. It does not remain, however, substantially, as a body, or a spirit; but it is nothing more than a certain affection of an evil quality, such as languor, for instance. There is not, to be sure, anything remaining which may be remitted whenever, as the Scripture says, “the Lord forgiveth all our iniquities.” But until that happens which immediately follows in the same passage, “Who healeth all thine infirmities, who redeemeth thy life from corruption,” there remains this concupiscence of the flesh in the body of this death. Now we are admonished not to obey its sinful desires to do evil: “Let not sin reign in your mortal body.” Still this concupiscence is daily lessened in persons of continence and increasing years, and most of all when old age makes a near approach. The man, however, who yields to it a wicked service, receives such great energies that, even when all his members are now failing through age, and those especial parts of his body are unable to be applied to their proper function, he does not ever cease to revel in a still increasing rage of disgraceful and shameless desire.