19 Men such as these you must never look at or associate with. Nor must you turn aside your heart unto words of evil lest the psalmist say to you: “You sit and speakest against your brother; you slander your own mother's son,” and lest you become as “the sons of men whose teeth are spears and arrows,” and as the man whose “words were softer than oil yet were they drawn swords.” The Preacher expresses this more clearly still when he says: “Surely the serpent will bite where there is no enchantment, and the slanderer is no better.” But you will say, 'I am not given to detraction, but how can I check others who are?'
If we put forward such a plea as this it can only be that we may “practise wicked works with men that work iniquity.” Yet Christ is not deceived by this device. It is not I but an apostle who says: “Be not deceived; God is not mocked.” “Man looks upon the outward appearance but the Lord looks upon the heart.” And in the proverbs Solomon tells us that as “the north wind drives away rain, so does an angry countenance a backbiting tongue.” It sometimes happens that an arrow when it is aimed at a hard object rebounds upon the bowman, wounding the would-be wounder, and thus, the words are fulfilled, “they were turned aside like a deceitful bow,” and in another passage: “whoso casts a stone on high casts it on his own head.” So when a slanderer sees anger in the countenance of his hearer who will not hear him but stops his ears that he may not hear of blood, he becomes silent on the moment, his face turns pale, his lips stick fast, his mouth becomes parched.
Wherefore the same wise man says: “meddle not with them that are given to detraction: for their calamity shall rise suddenly; and who knows the ruin of them both?” of him who speaks, that is, and of him who hears. Truth does not love corners or seek whisperers. To Timothy it is said, “Against an elder receive not an accusation suddenly; but him that sins rebuke before all, that others also may fear.” When a man is advanced in years you must not be too ready to believe evil of him; his past life is itself a defence, and so also is his rank as an elder.
Still, since we are but human and sometimes in spite of the ripeness of our years fall into the sins of youth, if I do wrong and you wish to correct me, accuse me openly of my fault: do not backbite me secretly. “Let the righteous smite me, it shall be a kindness, and let him reprove me; but let not the oil of the sinner enrich my head.” For what says the apostle? “Whom the Lord loves, he chastens, and scourges every son whom he receives.” By the mouth of Isaiah the Lord speaks thus: “O my people, they who call you happy cause you to err and destroy the way of your paths.” How do you help me by telling my misdeeds to others?
You may, without my knowing of it, wound some one else by the narration of my sins or rather of those which you slanderously attribute to me; and while you are eager to spread the news in all quarters, you may pretend to confide in each individual as though you had spoken to no one else. Such a course has for its object not my correction but the indulgence of your own failing. The Lord gives commandment that those who sin against us are to be arraigned privately or else in the presence of a witness, and that if they refuse to hear reason, the matter is to be laid before the church, and those who persist in their wickedness are to be regarded as heathen men and publicans.
Source: Letters (New Advent)