17 But if you then at least forgive him, or let go hatred from your heart, it is hatred from the heart I bid you forego, and not proper discipline. What if one who asks my pardon, be one who ought to be chastised by me! Do what you will, for I suppose that you love your child even when you chastise him. Thou regardest not his cries under the rod, because you are reserving for him his inheritance. This I say then, that you forego from your heart all hatred, when your enemy asks pardon of you.
But perhaps you will say, “he is playing false, he is pretending.” O you judge of another's heart, tell me your own father's thoughts, tell me your own thoughts yesterday. He asks and petitions for pardon; forgive, by all means forgive him. If you will not forgive him, it is yourself you hurt, not him, for he knows what he has to do. You are not willing to forgive your own fellow-servant; he will go then to your Lord, and say to Him, “Lord, I have prayed my fellow-servant to forgive me, and he would not; do Thou forgive me.”
Hath not the Lord power to release his servant's debts? So he, having obtained pardon from his Lord, returns loosed, while you remain bound. How bound? The time of prayer will come, the time must come for you to say, “Forgive us our debts, as we also forgive our debtors;” and the Lord will answer you, Thou wicked servant, when you owed Me so great a debt, you asked Me, and I forgave you; “should not you also have had compassion on your fellow-servant, even as I had pity on you?” These words are out of the Gospel, not of my own heart.
But if on being asked, you shall forgive him who begs for pardon, then you can say this prayer. And if you have not as yet the strength to love him in his violence, still you may offer this prayer, “Forgive us our debts, as we also forgive our debtors.” Let us pass on to the rest.
Source: Sermons on Selected Lessons of the New Testament (New Advent)