Augustine of Hippo
Catena Aurea · Matthäusevangelium 18
Therefore is the kingdom of heaven likened unto a certain king, which would take account of his servants. And when he had begun to reckon, one was brought unto him, which owed him ten thousand talents. But forasmuch as he had not to pay, his lord commanded him to be sold, and his wife, and children, and all that he had, and payment to be made. The servant therefore fell down, and worshipped him, saying, Lord, have patience with me, and I will pay thee all. Then the lord of that servant was moved with compassion, and loosed him, and forgave him the debt. But the same servant went out, and found one of his fellow-servants, which owed him an hundred pence: and he laid hands on him, and took him by the throat, saying, Pay me that thou owest. And his fellowservant fell down at his feet, and besought him, saying, Have patience with me, and I will pay thee all. And he would not: but went and cast him into prison, till he should pay the debt. So when his fellowservants saw what was done, they were very sorry, and came and told unto their lord all that was done. Then his lord, after that he had called him, said unto him, O thou wicked servant, I forgave thee all that debt, because thou desiredst me: Shouldest not thou also have had compassion on thy fellowservant, even as I had pity on thee? And his lord was wroth, and delivered him to the tormentors, till he should pay all that was due unto him. So likewise shall my heavenly Father do also unto you, if ye from your hearts forgive not every one his brother their trespasses.
(Serm. 83, 7.) or God says, Forgive, and ye shall be forgiven; (Luke 6:37.) I have first forgiven, forgive you then after Me; for if you forgive not, I will call you back, and will require again all that I had remitted to you. For Christ neither deceives nor is deceived; and He adds here, Thus will my heavenly Father do unto you, if ye from your hearts forgive not every one his brother their trespasses. It is better that you should cry out with your mouth, and forgive in your heart, than that you should speak smoothly, and be unrelenting in your heart For the Lord adds, From your hearts, to the end that though, out of affection you put him to discipline, yet gentleness should not depart out of your heart. What is more beneficial than the knife of the surgeon? He is rough with the sore that the man may be healed; should he be tender with the sore, the man were lost.