6 For indeed there is nothing equal to this virtue. Would you learn the power of this virtue? “Though Moses and Samuel stood before Me,” says God, “my soul would not regard them.” Nevertheless, those whom Moses and Samuel were not able to snatch away from God's wrath, this precept when observed was able to snatch away. Hence it is, that He continually exhorts those to whom He had spoken these things, saying, “Let none of you revengefully imagine evil against his brother in your heart,” and “let none of you think of his neighbour's malice.” It is not said merely, forego wrath; but retain it not in your mind; think not of it; part with all your resentment; do away the sore.
For you suppose that you are paying him back the injury; but you are first tormenting yourself, and setting up your rage as an executioner within you in every part, and tearing up your own bowels. For what can be more wretched than a man perpetually angry? And just as maniacs, who never enjoy tranquility, so also he who is resentful, and retains an enemy, will never have the enjoyment of any peace; incessantly raging, as he does, and daily increasing the tempest of his thoughts calling to mind his words and acts, and detesting the very name of him who has aggrieved him.
Do you but mention his enemy, he becomes furious at once, and sustains much inward anguish; and should he chance to get only a bare sight of him, he fears and trembles, as if encountering the worst evils. Yea, if he perceives any of his relations, if but his garment, or his dwelling, or street, he is tormented by the sight of them. For as in the case of those who are beloved, their faces, their garments, their sandals, their houses, or streets, excite us, the instant we behold them; so also should we observe a servant, or friend, or house, or street, or any thing else belonging to those we hate and hold our enemies, we are stung by all these things; and the strokes we endure from the sight of each one of them are frequent and continual.
Source: Homilies on the Statues (New Advent)