Objection 1. It would seem that backbiting is not a mortal sin. For no act of virtue is a mortal sin. Now, to reveal an unknown sin, which pertains to backbiting, as stated above (1, ad 3), is an act of the virtue of charity, whereby a man denounces his brother's sin in order that he may amend: or else it is an act of justice, whereby a man accuses his brother. Therefore backbiting is not a mortal sin.
Objection 2. Further, a gloss on Proverbs 24:21, "Have nothing to do with detractors," says: "The whole human race is in peril from this vice." But no mortal sin is to be found in the whole of mankind, since many refrain from mortal sin: whereas they are venial sins that are found in all. Therefore backbiting is a venial sin.
Objection 3. Further, Augustine in a homily On the Fire of Purgatory [Serm. civ in the appendix to St. Augustine's work] reckons it a slight sin "to speak ill without hesitation or forethought." But this pertains to backbiting. Therefore backbiting is a venial sin.
On the contrary, It is written (Romans 1:30): "Backbiters, hateful to God," which epithet, according to a gloss, is inserted, "lest it be deemed a slight sin because it consists in words."
I answer that, As stated above (Question 72, Article 2), sins of word should be judged chiefly from the intention of the speaker. Now backbiting by its very nature aims at blackening a man's good name. Wherefore, properly speaking, to backbite is to speak ill of an absent person in order to blacken his good name. Now it is a very grave matter to blacken a man's good name, because of all temporal things a man's good name seems the most precious, since for lack of it he is hindered from doing many things well. For this reason it is written (Sirach 41:15): "Take care of a good name, for this shall continue with thee, more than a thousand treasures precious and great." Therefore backbiting, properly speaking, is a mortal sin. Nevertheless it happens sometimes that a man utters words, whereby someone's good name is tarnished, and yet he does not intend this, but something else. This is not backbiting strictly and formally speaking, but only materially and accidentally as it were. And if such defamatory words be uttered for the sake of some necessary good, and with attention to the due circumstances, it is not a sin and cannot be called backbiting. But if they be uttered out of lightness of heart or for some unnecessary motive, it is not a mortal sin, unless perchance the spoken word be of such a grave nature, as to cause a notable injury to a man's good name, especially in matters pertaining to his moral character, because from the very nature of the words this would be a mortal sin. And one is bound to restore a man his good name, no less than any other thing one has taken from him, in the manner stated above (Question 62, Article 2) when we were treating of restitution.
Reply to Objection 1. As stated above, it is not backbiting to reveal a man's hidden sin in order that he may mend, whether one denounce it, or accuse him for the good of public justice.
Reply to Objection 2. This gloss does not assert that backbiting is to be found throughout the whole of mankind, but "almost," both because "the number of fools is infinite," [Ecclesiastes 1:15 and few are they that walk in the way of salvation, [Cf. Matthew 7:14] and because there are few or none at all who do not at times speak from lightness of heart, so as to injure someone's good name at least slightly, for it is written (James 3:2): "If any man offend not in word, the same is a perfect man."
Reply to Objection 3. Augustine is referring to the case when a man utters a slight evil about someone, not intending to injure him, but through lightness of heart or a slip of the tongue.
Source: Summa Theologica (New Advent)