For he shall have judgment without mercy, that has showed no mercy; and mercy triumphs over judgment.
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Augustine of Hippo
He who judges without mercy will be judged without mercy. And in this sense only is the "same measure" to be understood, that the mercy which he did not show will not be shown to him, and that the judgment which he makes will be eternal, even though the thing judged cannot be eternal.
For judgment without mercy It is an admonition to them to fulfil, as he said before, the royal precepts of the love of God and of our neighbour, which cannot be without being merciful to others. Blessed are the merciful, for they shall obtain mercy. (Matthew v. 7.)
And mercy exalteth itself above judgment. Some understand this as a confirmation of God's infinite mercies, out of Psalm cxliv. 9. where it is said that his "mercies are over all his works "that is, though all his perfections be equally infinite, yet he is pleased to deal with sinners rather according to the multitude of his mercies than according to the rigour of his justice. Others expound these words of the mercy which men show to one another, and that he exhorts them to mercy, as a most powerful means to find mercy; and the merciful works done to others will be beneficial to them, and make them escape when they come to judgment. (Witham)
Similar to this are the words of old Tobias to his son: "Alms deliver from all sin...
Mercy is the highest art and the shield of those who practice it. It is the friend of God, standing always next to him and freely blessing whatever he wishes. It must not be despised by us. For in its purity it grants great liberty to those who respond to it in kind. It must be shown to those who have quarreled with us, as well as to those who have sinned against us, so great is its power. It breaks chains, dispels darkness, extinguishes fire, kills the worm and takes away the gnashing of teeth. (Mk 9:44-48) By it the gates of heaven open with the greatest of ease. In short, mercy is a queen which makes men like God.
If we forgive others the sins which they have committed against us and give alms to the poor and needy among us, then God's mercy will deliver us from judgment. But if, on the other hand, we are not well-disposed toward those around us, we shall receive the condemnation handed out to the wicked servant, along with the retribution which is mentioned in the Lord's Prayer. For there we ask God to forgive us as we forgive those who have sinned against us, (Mt 6:12) but if we do not forgive them, we shall not be forgiven either.