I answer that, According to the Apostle (Rm. 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]).