7 This conflict none experience in themselves, save such as war on the side of the virtues, and war down the vices: nor does any thing storm the evil of lust, save the good of Continence. But there are, who, being utterly ignorant of the law of God, account not evil lusts among their enemies, and through wretched blindness being slaves to them, over and above think themselves also blessed, by satisfying them rather than taming them. But whoso through the Law have come to know them, (“For through the Law is the knowledge of sin,” and, “Lust,” says he, “I knew not, unless the Law should say, You shall not lust after,” and yet are overcome by their assault, because they live under the Law, whereby what is good is commanded, but not also given: they live not under Grace, which gives through the Holy Spirit what is commanded through the Law: unto these the Law therefore entered, that in them the offense might abound.
The prohibition increased the lust, and made it unconquered: that there might be transgression also, which without the Law was not, although there was sin, “For where there is not Law, neither is there transgression.” Thus the Law, Grace not helping, forbidding sin, became over and above the strength of sin; whence the Apostle says, “The Law is the strength of sin.” Nor is it to be wondered at, that man's weakness even from the good Law added strength to evil, while it trusts to fulfill the Law itself of its own strength.
Forsooth being ignorant of the righteousness of God, which He gives unto the weak, and wishing to establish his own, of which the weak is void, he was not made subject to the righteousness of God, reprobate and proud. But if the Law, as a schoolmaster, lead unto Grace one made an offender, as though for this purpose more grievously wounded, that he may desire a Physician; against the baneful sweetness, whereby lust prevailed, the Lord gives a sweetness that works good, that by it Continence may the more delight, and “our land gives her fruit,” whereby the soldier is fed, who by the help of the Lord wars down sin.
Source: On Continence (New Advent)