God requires two things of us here: to condemn ourselves for our sins, and to forgive others.

And we are to do the former for the sake of the latter, that this may become easier (for he who considers his own sins is more indulgent to his fellow-servant).

And we are to forgive not merely with the lips, but from the heart.

Let us not then thrust the sword into ourselves by being revengeful.

For what grief has he who has grieved you inflicted upon you which is as bad as that which you will work unto yourself by keeping your anger in mind, and drawing upon yourself the sentence from God to condemn you?

If you are watchful, and keep yourself under control, the evil will come round upon his head, and it will be he that will suffer harm.

But if you should carry on being indignant and displeased, then you yourself will undergo the harm – not from him, but from yourself.

Say not then that he insulted you, and slandered you, and did unto you ills beyond number; for the more you say, so much the more do you declare him a benefactor.

For he has given you an opportunity to wash away your sins – so that the greater the injuries he has done you, so much more has he become for you a cause of a greater remission of sins.

For if we be willing, no one shall be able to injure us, but even our enemies shall benefit us in the greatest degree.

And why do I speak of men? For what can be more wicked than the devil? Yet nevertheless, even hence have we a great opportunity of approving ourselves, as the case of Job shows us.

But if even the devil has become a cause of obtaining crowns, why are you afraid of a man as an enemy?

See then how much you gain, bearing meekly the spiteful acts of your enemies.

First and greatest, you obtain deliverance from sins;

secondly, fortitude and patience;

thirdly, mildness and benevolence;

[…] fourthly, to be free from anger continually, to which nothing can be equal.

For of him that is free from anger, it is quite clear that he is delivered also from the despondency arising from anger, and will not spend his life on vain labours and sorrows.

For he that does not know how to hate, likewise does not know how to grieve, but will enjoy pleasure, and ten thousand blessings.

Accordingly, we punish ourselves by hating others, even as on the other hand we benefit ourselves by loving them.

John Chrysostom (c.347-407): Homily 61,5 on St Matthew’s Gospel.