Therefore, is thy brother a sinner? Then cover his sin and pray for him. Dost thou publish his sins, then truly thou art not a child of your merciful Father; for otherwise thou wouldst be also as he, merciful. It is certainly true that we cannot show as great mercy to our neighbor, as God has to us; but it is the true work of the devil that we do the very opposite of mercy, which is a sure sign that there is not a grain of mercy in us.

The Complete Sermons of Martin Luther: Volume 4: (ed. 2000)