13 Let us then return to the words, which I had set forth out of the Apostle as obscure, and we shall now see them to be plain. For this I had set forth, that the Apostle did not say, “Walk in the Spirit, and you shall not have the lusts of the flesh;” because we must necessarily have them. Why then did he not say, “You shall not do the lusts of the flesh”? Because we do them; for we do lust. The very lusting, is doing. But the Apostle says, “Now it is no more I that do it, but sin that dwells in me.” What then have you to beware of?
This doubtless, that you fulfil them not. A damnable lust has risen up, it has risen, made its suggestion; let it not be heard. It burns, and is not quieted, and you would that it should not burn. Where then is, “So that you cannot do the things that you would”? Do not give it your members. Let it burn without effect, and it will spend itself. In you then these lusts are done. It must be confessed, they are done. And therefore he said, “You shall not fulfil.” Let them not then be fulfilled.
You have determined to do, you have fulfilled. For you have fulfilled it, if you determine upon committing adultery, and dost not commit it, because no place has been found, because no opportunity is given, because, it may be, she for whom you seem to be disturbed is chaste; lo, now she is chaste, and you are an adulterer. Why? Because you have fulfilled lusts. What is, “have fulfilled”? have determined in your mind upon committing adultery. If now, which God forbid, your members too have wrought, you have fallen down headlong into death.
Source: Sermons on Selected Lessons of the New Testament (New Advent)