From “Homily 9 on the Acts of the Apostles” (400).

“Repent,” Peter says, “and be converted… that your sins, may be blotted out… so shall the times of refreshing come from the presence of the Lord… Still you may yet obtain pardon… Unto you God sent his son Jesus to bless you.” Let us then also imitate Christ. Let us cast out that spirit of murder and enmity. It is not enough not to retaliate (for even in the old dispensation this was exemplified). Rather let us do all as we would for bosom-friends, as we would for ourselves, so too for those who have injured us. We are followers of Christ, we are his disciples, who after being crucified, sets everything in action on behalf of his murderers, and sends out his apostles to this end…

Those acted not only unjustly, but impiously; for he was their benefactor; he had done no evil, and they crucified him. And for what reason? For the sake of their reputation. But he himself made them objects of reverence. “The scribes and the pharisees sit in Moses’ seat; all therefore whatsoever they bid you observe, that do ye, but after their works do ye not” (Matt. 23:2)… When he might have destroyed them, he saves them. Let us then imitate him, and let no one be an enemy, no one a foe, except to the devil.

Not a little does the habit of not swearing contribute to this end: I mean to the not giving way to wrath: and by not giving way to wrath, we shall not have an enemy either. Lop off the oaths of a man, and you have clipped the wings of his anger, you have smothered all his passion…

Let all men see, that of those who assemble in this Church not one is a swearer. By this also let us become manifest, and not by our creed alone; let us have this mark also to distinguish us both from the Gentiles and from all men. Let us receive it as a seal from heaven, that we may everywhere be seen to be the King’s own flock…

Let us then be known by speaking like the apostles; by speaking like the angels. If anyone bid you swear tell him, Christ has spoken, and I do not swear. This is enough to make a way for all virtue to come in. It is a gate to religion, a high road leading to the philosophy of piety; a kind of training-school. These things let us observe, that we may obtain also the future blessings, through the grace and mercy of our Lord Jesus Christ.

St. John Chrysostom (ca. 347-407) was Archbishop of Constantinople, and one of the greatest preachers of his era. He is traditionally counted among the Four Great Doctors of the Eastern Church.  His Homilies on the Acts of the Apostles were preached during his ministry in Constantinople, at a time of great public peril. His feast day is September 13.