6 Seeing then that not even where we “know nothing against ourselves,” can we be clean from accusations, and where we do any thing good, but do it not of a right mind, we are liable to punishment; consider how vastly men are deceived in their judgments. For all these matters are not be come at by men, but by the unsleeping Eye alone: and though we may deceive men, our sophistry will never avail against Him. Say not then, darkness is around me and walls; who sees me? For He who by Himself formed our hearts, Himself knows all things. “For darkness is no darkness with Him.” And yet he who is committing sin, well says, “Darkness is around me and walls;” for were there not a darkness in his mind he would not have cast out the fear of God and acted as he pleased. For unless the ruling principle be first darkened, the entrance of sin without fear is a thing impossible. Say not then, who sees me? For there is that “pierces even unto soul and spirit, joints and marrow;” but you see not yourself nor can you pierce the cloud; but as if you had a wall on all sides surrounding you, you are without power to look up unto the heaven.
For whatsoever sin you will, first let us examine, and you shall see that so it is engendered. For as robbers and they who dig through walls when they desire to carry off any valuable thing, put out the candle and then do their work; so also does men's perverse reasoning in the case of those who are committing sin. Since in us also surely there is a light, the light of reason, ever burning. But if the spirit of wickedness coming eagerly on with its strong blast quench that flame, it straightway darkens the soul and prevails against it, and despoils it straightway of all that is laid up therein. For when by unclean desire the soul is made captive, even as a cloud and mist the eyes of the body, so that desire intercepts the foresight of the mind, and suffers it to see nothing at any distance, either precipice, or hell, or fear; but thenceforth, having that deceit as a tyrant over him, he comes to be easily vanquished by sin; and there is raised up before his eyes as it were a wall without windows, which suffers not the ray of righteousness to shine in upon the mind, the absurd conceits of lust enclosing it as with a rampart on all sides. And from that time forward the unchaste woman is everywhere meeting him: standing present before his eyes, before his mind, before his thoughts. And as the blind, although they stand at high noon beneath the very central point of the heaven, receive not the light, their eyes being fast closed up; just so these also, though ten thousand doctrines of salvation sound in their ears from all quarters, having their soul preoccupied with this passion stop their ears against such discourses. And they know it well who have made the trial. But God forbid that you should know it from actual experience.
Source: Homilies on First Corinthians (New Advent)