8 Such soldiers the Apostolic trumpet enkindles for battle with that sound, “Therefore let not,” says he, “sin reign in your mortal body to obey its lusts; nor yield your members weapons of unrighteousness unto sin; but yield yourselves unto God, as living in place of dead, and your members weapons of righteousness unto God. For sin shall not rule over you. For you are not under the law, but under Grace.” And in another place, “Therefore,” says he, “brethren, we are debtors, not to the flesh, to live after the flesh.
For if you shall live after the flesh, you shall die; but if by the Spirit you shall mortify the deeds of the flesh, you shall live. For as many as are led by the Spirit of God, these are sons of God.” This therefore is the business in hand, so long as this our mortal life under Grace lasts, that sin, that is the lust of sin, (for this he in this place calls by the name of sin,) reign not in this our mortal body. But it is then shown to reign, if obedience be yielded to its desires.
There is therefore in us lust of sin, which must not be suffered to reign; there are its desires, which we must not obey, lest obeying it reign over us. Wherefore let not lust usurp our members, but let Continence claim them for herself; that they be weapons of righteousness unto God, that they be not weapons of unrighteousness unto sin; for thus sin shall not rule over us. For we are not under the Law, which indeed commands what is good yet gives it not: but we are under Grace, which, making us to love that which the Law commands, is able to rule over the free.
Source: On Continence (New Advent)