16 What beyond—that you should not swear nor curse; that you should not seek again your goods when taken from you; that, when you receive a buffet, you should give your other cheek to the smiter; that you should forgive a brother who sins against you, not only seven times, but seventy times seven times, but, moreover, all his sins altogether; that you should love your enemies; that you should offer prayer for your adversaries and persecutors? Can you accomplish these things unless you maintain the steadfastness of patience and endurance?
And this we see done in the case of Stephen, who, when he was slain by the Jews with violence and stoning, did not ask for vengeance for himself, but for pardon for his murderers, saying, “Lord, lay not this sin to their charge.” It behooved the first martyr of Christ thus to be, who, fore-running the martyrs that should follow him in a glorious death, was not only the preacher of the Lord's passion, but also the imitator of His most patient gentleness. What shall I say of anger, of discord, of strife, which things ought not to be found in a Christian?
Let there be patience in the breast, and these things cannot have place there; or should they try to enter, they are quickly excluded and depart, that a peaceful abode may continue in the heart, where it delights the God of peace to dwell. Finally, the apostle warns us, and teaches, saying: “Grieve not the Holy Spirit of God, in whom you are sealed unto the day of redemption. Let all bitterness, and anger, and wrath, and clamour, and blasphemy, be put away from you.” For if the Christian have departed from rage and carnal contention as if from the hurricanes of the sea, and have already begun to be tranquil and meek in the harbour of Christ, he ought to admit neither anger nor discord within his breast, since he must neither return evil for evil, nor bear hatred.
Source: The Treatises of Cyprian (New Advent)