This very important teaching of St. Francis de Sales should be printed framed, and kept on the wall of every Catholic home.  

Public, notorious sinners may be spoken of freely, provided always even then that a spirit of charity and compassion prevail, and that you do not speak of them with arrogance or presumption, or as though you took pleasure in the fall of others. To do this is the sure sign of a mean ungenerous mind. And, of course, you MUST speak freely in condemnation of the professed enemies of God and His Church, heretics and schismatics,--it is true charity to point out the wolf wheresoever he creeps in among the flock. Most people permit themselves absolute latitude in criticising and censuring rulers, and in calumniating nationalities, according to their own opinions and likings. But do you avoid this fault; it is displeasing to God, and is liable to lead you into disputes and quarrels. When you hear evil of any one, cast any doubt you fairly can upon the accusation; or if that is impossible, make any available excuse for the culprit; and where even that may not be, be yet pitiful and compassionate, and remind those with whom you are speaking that such as stand upright do so solely through God's Grace. Do your best kindly to check the scandal-bearer, and if you know anything favourable to the person criticised, take pains to mention it.