"If you neglect to [reprove him], you are worse than he is."

"But what about those who have suffered an injury, what ought they to do?  What we have heard in today’s gospel:  If your brother has sinned against you, reprove him between yourself and him alone.  If you neglect to do so, you are worse than he is [(Si neglexeris, pejor es)].  He has done wrong, and by doing wrong has inflicted a grave wound on himself; are you going to ignore your brother’s wound?  You see that he’s on the point of being lost, or is already lost, and are you not going to bother?  You’re worse by keeping silent than he was by noisily abusing you."

     St. Augustine, Sermon 82.4.7 (408), trans. Edmund Hill, O.P. (WSA III.iii, Sermons III (51-94) on the New Testament (Brooklyn:  New City Press, 1991), 372) =PL 38, cols. 508-509 (506-514).  Cf. the "Si neglexeris corrigere, peior eo factus es qui peccavit" cited by Aquinas at ST II-II.33.2.Sed contra (from, supposedly, the De verbo Dom. xvi.4, but which vol. 34 of the Blackfriars edition, p. 279n5, also traces to Sermon 82).

     St. Augustine goes on to distinguish between the response to private and public sin (10):

Those sins, then, are to be rebuked in front of everybody which are committed in front of everybody.  Those which are committed less publicly are to be rebuked less publicly.  Distinguish between the occasions. . . .

(And do it for the right motives, i.e. the recovery/salvation of the sinner.)