Objection 1. It would seem that the listener who suffers a backbiter does not sin grievously. For a man is not under greater obligations to others than to himself. But it is praiseworthy for a man to suffer his own backbiters: for Gregory says (Hom. ix, super Ezech): "Just as we ought not to incite the tongue of backbiters, lest they perish, so ought we to suffer them with equanimity when they have been incited by their own wickedness, in order that our merit may be the greater." Therefore a man does not sin if he does not withstand those who backbite others.
Objection 2. Further, it is written (Sirach 4:30): "In no wise speak against the truth." Now sometimes a person tells the truth while backbiting, as stated above (1, ad 3). Therefore it seems that one is not always bound to withstand a backbiter.
Objection 3. Further, no man should hinder what is profitable to others. Now backbiting is often profitable to those who are backbitten: for Pope Pius [St. Pius I] says [Append. Grat. ad can. Oves, caus. vi, qu. 1: "Not unfrequently backbiting is directed against good persons, with the result that those who have been unduly exalted through the flattery of their kindred, or the favor of others, are humbled by backbiting." Therefore one ought not to withstand backbiters.
On the contrary, Jerome says (Ep. ad Nepot. lii): "Take care not to have an itching tongue, nor tingling ears, that is, neither detract others nor listen to backbiters."
I answer that, According to the Apostle (Romans 1:32), they "are worthy of death... not only they that" commit sins, "but they also that consent to them that do them." Now this happens in two ways. First, directly, when, to wit, one man induces another to sin, or when the sin is pleasing to him: secondly, indirectly, that is, if he does not withstand him when he might do so, and this happens sometimes, not because the sin is pleasing to him, but on account of some human fear.
Accordingly we must say that if a man list ens to backbiting without resisting it, he seems to consent to the backbiter, so that he becomes a participator in his sin. And if he induces him to backbite, or at least if the detraction be pleasing to him on account of his hatred of the person detracted, he sins no less than the detractor, and sometimes more. Wherefore Bernard says (De Consid. ii, 13): "It is difficult to say which is the more to be condemned the backbiter or he that listens to backbiting." If however the sin is not pleasing to him, and he fails to withstand the backbiter, through fear negligence, or even shame, he sins indeed, but much less than the backbiter, and, as a rule venially. Sometimes too this may be a mortal sin, either because it is his official duty to cor. rect the backbiter, or by reason of some consequent danger; or on account of the radical reason for which human fear may sometimes be a mortal sin, as stated above (Question 19, Article 3).
Reply to Objection 1. No man hears himself backbitten, because when a man is spoken evil of in his hearing, it is not backbiting, properly speaking, but reviling, as stated above (1, ad 2). Yet it is possible for the detractions uttered against a person to come to his knowledge through others telling him, and then it is left to his discretion whether he will suffer their detriment to his good name, unless this endanger the good of others, as stated above (Question 72, Article 3). Wherefore his patience may deserve commendation for as much as he suffers patiently being detracted himself.
But it is not left to his discretion to permit an injury to be done to another's good name, hence he is accounted guilty if he fails to resist when he can, for the same reason whereby a man is bound to raise another man's ass lying "underneath his burden," as commanded in Deuteronomy 21:4 [Exodus 23:5].
Reply to Objection 2. One ought not always to withstand a backbiter by endeavoring to convince him of falsehood, especially if one knows that he is speaking the truth: rather ought one to reprove him with words, for that he sins in backbiting his brother, or at least by our pained demeanor show him that we are displeased with his backbiting, because according to Proverbs 25:23, "the north wind driveth away rain, as doth a sad countenance a backbiting tongue."
Reply to Objection 3. The profit one derives from being backbitten is due, not to the intention of the backbiter, but to the ordinance of God Who produces good out of every evil. Hence we should none the less withstand backbiters, just as those who rob or oppress others, even though the oppressed and the robbed may gain merit by patience.
Source: Summa Theologica (New Advent)