"If any one sin, and hear the voice of one swearing, and is a witness either because he himself hath seen, or is privy to it: if he do not utter it, he shall bear his iniquity." (Leveticus. 5:1)
Pope St. Felix III (483-492): Not to oppose error, is to approve it, and indeed to neglect to confound evil men, when we can do it, is no less a sin than to encourage them.