See in what way gentleness is destroyed, when evil thoughts gnaw at it. This is why there is such a dog, barking mightily: we have also sling and stone (ye know what I mean): we have also spear and enclosure and cattle-fold: let us guard our thoughts unhurt. If the dog be gentle (σαίνῃ) with the sheep, but savage against those without, and keep vigilant watch, this is the excellence of a dog: and, be he ever so famished, not to devour the sheep; be he ever so full, not to spare the wolves.
Such too is anger meant to be: however provoked, not to forsake gentleness; however at quiet, to be on the alert against evil thoughts: to acknowledge the friend, and not for any beating forsake him, and for all his caressing, to fly at the intruder. The devil uses caressing full oft: let the dog know at sight that he is an intruder. So also let us caress (σαίνωμεν) Virtue, though she put us to pain, and show our aversion to Vice, though she give us pleasure. Let us not be worse than the dogs, which, even when whipped and throttled, do not desert their master: but if the stranger also feed them, even so they do hurt.
There are times when anger is useful; but this is when it barks against strangers. What means it, “Whosoever is angry with his brother without a cause?” It means, Stand not up in your own quarrel, neither avenge yourself: if you see another suffering deadly wrong, stretch out your hand to help him. This is no longer passion, when you are clear of all feeling for yourself alone. David had gotten Saul into his power, and was not moved by passion, did not thrust the spear into him, the enemy he had in his power; but took his revenge upon the Devil. Moses, when he saw a stranger doing an injury, even slew him: but when one of his own people, he did not so: them that were brethren he would have reconciled; the others not so.
That “most meek” Moses, as Scripture witnesses of him, see how he was roused! But not so, we: on the contrary, where we ought to show meekness, no wild beast so fierce as we: but where we ought to be roused, none so dull and sluggish. (Hom. vi. de laud. Pauli, ad fin.) On no occasion do we use our faculties to the purpose they were meant for: and therefore it is that our life is spent to no purpose. For even in the case of implements; if one use them, one instead of other, all is spoilt: if one take his sword, and then, where he should use it and cut with it, uses only his hand, he does no good: again, where he should use his hand, by taking the sword in hand he spoils all.
In like manner also the physician, if where he ought to cut, he cuts not, and where he ought not, he does cut, mars all. Wherefore, I beseech you, let us use the thing (τᾥ πράγματι) at its proper time. The proper time for anger is never, where we move in our own quarrel: but if it is our duty to correct others, then is the time to use it, that we may by force deliver others. (Hom. in Matt. xvi. §7.) So shall we both be like God, always keeping a spirit free from wrath, and shall attain unto the good things that are to come, through the grace and loving-kindness of our Lord Jesus Christ, with whom, to the Father and the Holy Ghost together, be glory, dominion, and honor, now and evermore, world without end. Amen.
Source: Homilies on Acts (New Advent)