8 Be not willing then to be overcome when you fight. See what kind of war, what kind of battle, what kind of strife he has set forth, within, within your own self. “The flesh lusts against the Spirit.” If the Spirit lust not also against the flesh, commit adultery. But if the Spirit lust against the flesh, I see a struggle, I do not see a victory, it is a contest. “The flesh lusts against the Spirit.” Adultery has its pleasure. I confess that it has its pleasure. But, “The Spirit lusts against the flesh:” Chastity too has its pleasure.
Therefore let the Spirit overcome the flesh; or by all means not be overcome by the flesh. Adultery seeks the darkness, chastity desires the light. As you would wish to appear to others, so live; as you would wish to appear to men, even when beyond the eyes of men so live; for He who made you, even in the darkness sees you. Why is chastity praised publicly by all? Why do not even adulterers praise adultery? “Whoso” then “seeks the truth, comes to the light.” But adultery has its pleasure.
Be it contradicted, resisted, opposed. For it is not so that you have nothing wherewith to fight. Your God is in you, the good Spirit has been given to you. And notwithstanding this flesh of ours is permitted to lust against the spirit by evil suggestions and real delights. Be that secured which the Apostle says, “Let not sin reign in your mortal body.” He did not say, “Let it not be there.” It is there already. And this is called sin, because it has befallen us through the wages of sin.
For in Paradise the flesh did not lust against the spirit, nor was there this struggle there, where was peace only; but after the transgression, after that man was loth to serve God and was given up to himself; yet not so given up to himself as that he could so much as possess himself; but possessed by him, by whom deceived; the flesh began to lust against the Spirit. Now it is in the good that it lusts against the Spirit; for in the bad it has nothing to lust against. For there does it lust against the Spirit, where the Spirit is.
Source: Sermons on Selected Lessons of the New Testament (New Advent)