6 You are a man, the dust of the earth, clay, kinsman of the clod; you are the son of the race of beasts. If you know not your honour; separate your soul from animals, by works and not by words. If you love derision, you are altogether as Satan; and if you mock at your fellow, you are the mouth of the Devil; if against defects and flaws, in (injurious) names you delight, Satan is not in creation but his place you have seized by force. Get you far, O man, from this; for it is altogether hurtful; and if you desire to live well, sit not with the scorner, lest you become the partner of his sin and of his punishment.
Hate mockery which is altogether (the cause of weeping), and mirth which is (the cause of) cleansing. And if you should hear a mocker by chance, when you are not desiring it, sign yourself with the cross of light, and hasten from thence like an antelope. Where Satan lodges, Christ will in nowise dwell; a spacious dwelling for Satan is the man that mocks at his neighbour; a palace of the Enemy is the heart of the mocker. Satan does not desire to add any other evil to it. Mockery is sufficient for him to supply the place of all.
Neither his belly nor yet his purse can (the sinner) fill with that sin of his. By his laughter is the wretch despoiled, and he knows not nor does he perceive it. For his wound, there is no cure; for his sickness, there is no healing; his pain, admits no remedy; and his sore, endures no medicine. I desire not with such a one to put forth my tongue to reprove him: enough for him is his own shame; sufficient for him is his boldness. Blessed is he that has not heard him; and blessed is he that has not known him. Be it far from you, O Church, that he should enter you, that evil leaven of Satan!
Source: On Admonition and Repentance (New Advent)