8 If so be you give a gift in pride, this is not of God. If you are lifted up by reason of your knowledge, you have denied the grace of God. If you are poor and proud, lo! Your end is in your torment. If you are haughty and needy, lo! Your need is toward your destruction. If you are sick and criest out, lo! Your trouble is full of harm. If you are in need of food, yet your mind longs for riches; your distress is with the poor, but your torment with the rich. If you shall look unchastely, and shall desire your neighbour's wife, lo!
Your portion shall be with the adulterers, and your hell with the fornicators. Let your own fountain be for yourself, and drink waters from your well. Let your fountains be for yourself alone, and let not another drink with you. Require purity of your body as you require of your yoke-fellow. You would not have her commit lewdness, the wife of your youth, with another man; commit not lewdness with another woman, the wife of a different husband. Let the defilement of her be hateful in your eyes; keep aloof from it altogether.
Chastity beseems the wife; purity is as her adornment; law becomes the husband; justice is the crown for his head. Desire not the bed of your neighbour lest another desire your bed. Preserve purity in your marriage, that your marriage may be holy. His conscience reproves the man, who corrupts the wife of his neighbour. He fears, and deceives through terror, whoso has engaged in fornication. Darkness is dearer to him than light, whose manner of life is not pure. Every hour he stands in dread, who commits adultery secretly.
The adulterer is also a thief who breaks into houses in darkness. The very place reproves him, where he does the evil and wickedness. He enters the chamber and sins; in the darkness he does his will. The time will come when it shall be disclosed, when his secret deeds shall be manifested. With what eyes do you look towards God in prayer? What hands do you raise when you ask pardon? Be ashamed and dismayed for yourself, that you are void of understanding. If when your neighbour see you, you are ashamed and dismayed, how much more should you be ashamed before God Who sees all?
You are like the sow, your companion, that wallows altogether in mire. Even in seeing, you may sin, if your mind is not watchful; and in hearing you may transgress, if you do not guard your hearing. The fornicator's heart waxes wanton through speech that is full of uncleanness. The passion hidden in the mind, sight and hearing awaken it.
Source: On Admonition and Repentance (New Advent)