St. John Chrysostom
http://www.fordham.edu/halsall/source/chrysostom-Jєωs6.html#HOMILY_I
I could have said more than this, but to keep you from forgetting what I have said, I shall bring my homily to an end here with the words of Moses: "I call heaven and earth to witness against you". If any of you, whether you are here present or not, shall go to the spectacle of the Trumpets, or rush off to the ѕуηαgσgυє, or go up to the shrine of Matrona, or take part in fasting, or share in the Sabbath, or observe any other Jєωιѕн ritual great or small, I call heaven and earth as my witnesses that I am guiltless of the blood of all of you.
(2) These words will stand by your side and mine on the day of our Lord Jesus Christ. If you heed them, they will bring you great confidence; if you heed them not or conceal anyone who dares to do those things, my words shall stand against you as bitter accusations. "For I have not shrunk from declaring to you the whole counsel of God".
(3) I have deposited the money with the bankers. It remains for you to increase the deposit and to use the profit from my words for the salvation of your brothers. Do you find it an oppressive burden to denounce those who commit these sins? It is an oppressive burden to remain silent. For this silence makes you an enemy to God and brings destruction both to you who conceal such sinners and to those whose sins go unrevealed. How much better it is to become hateful to our fellow servants for saving them to provoke God's anger against yourselves. Even if your fellow servant be vexed with you now, he will not be able to harm you but will be grateful later on for his cure. But if you seek to win your fellow servant's favor, if you remain silent and hurt him by concealing his sin, God will exact from you the ultimate penalty. Your silence will make God your foe and will hurt your brother; if you denounce him and reveal his sin, you will make God propitious and benefit your brother and you will gain as a friend one who was crazed but who learned from experience that you served him well.
(4) Do not think, then, that you are doing your brothers a favor if you should see them pursuing some absurdity and should fail to accuse them with all zeal. If you lose a cloak, do you not consider as your foe not only the one who stole it but also the man who knew of the theft and refused to denounce the thief? Our common Mother (the Church) has lost not a cloak but a brother. The devil stole him and now holds him in Judaism. You know who stole him; you know him who was stolen. Do you see me lighting, as it were, the lamp of my instruction and searching everywhere in my grief? And do you stand silent, refusing to denounce him? What excuse will you have? Will the Church not reckon you among her worst enemies? Will she not consider you a foe and destroyer?
(5) Heaven forbid that anyone who hears my words of advice should commit such a sin as to betray the brother for whom Christ died. Christ poured out his blood on his account. Are you too reluctant to utter a word on this account?