3 In order then that we may be able well to do and to fulfil what we have been admonished of today, let us first persuade ourselves to this, above all things to have no hate. For when there is no beam in your own eye, you see rightly whatever may be in your brother's eye; and art uneasy, till you cast out of your brother's eye what you see to hurt it. The light that is in you, does not allow you to neglect your brother's light. Whereas if you hate, and would correct him, how do you improve his light, when you have lost your own light?
For the same Scripture, where it is written, “Whosoever hates his brother is a murderer,” has expressly told us this also. “He that hates his brother is in darkness even until now.” Hatred then is darkness. Now it cannot but be, that he who hates another, should first injure himself. For him he endeavours to hurt outwardly, he lays himself waste inwardly. Now in proportion as our soul is of more value than our body, so much the more ought we to provide for it, that it be not hurt.
But he that hates another, does hurt his own soul. And what would he do to him whom he hates? What would he do? He takes away his money, can he take his faith away? He wounds his good fame, can he wound his conscience? Whatever injury he does, is but external; now observe what his injury to himself is? For he who hates another is an enemy to himself within. But because he is not sensible of what harm he is doing to himself, he is violent against another, and that the more dangerously, that he is not sensible of the evil he is doing to himself; because by this very violence he has lost the power of perception.
You are violent against your enemy; by this violence of yours he is spoiled, and you are wicked. Great is the difference between the two. He has lost his money, you have lost your innocence. Ask which has suffered the heavier loss? He has lost a thing that was sure to perish, and you have become one who must now perish yourself.
Source: Sermons on Selected Lessons of the New Testament (New Advent)