Whether backbiting is a mortal sin?

❌ 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 (Article [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 Prov. 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.