The Lord puts the parable of the unforgiving debtor before us that we may learn from it. He has no desire for us to die, so he warns us: This is how your heavenly Father will deal with you if you, any of you, fail to forgive your brother or sister from your heart.
Take notice now, for clearly this is no idle warning. The fulfillment of this command calls for the most vigorous obedience. We are all in debt to God, just as other people are in debt to us. Is there anyone who is not God’s debtor? Only a person in whom no sin can be found. And is there anyone who has no brother or sister in his debt? Only if there be someone who has never suffered any wrong. Do you think anyone can be found in the entire human race who has not in turn wronged another in some way, incurring a debt to that person? No, all are debtors, and have others in debt to them. Accordingly, God who is just has told you how to treat your debtor, because he means to treat his in the same way.
There are two works of mercy which will set us free. They are briefly set down in the gospel in the Lord’s own words: Forgive and you will be forgiven, and Give and you will receive. The former concerns pardon, the latter generosity.
As regards pardon he says: “Just as you want to be forgiven, so someone is in need of your forgiveness.” Again, as regards generosity, consider when a beggar asks you for something that you are a beggar too in relation to God. When we pray we are all beggars before God. We are standing at the door of a great householder, or rather, lying prostrate, and begging with tears. We are longing to receive a gift—the gift of God himself. What does a beggar ask of you? Bread. And you, what do you ask of God, if not Christ who said: I am the living bread that has come down from heaven? Do you want to be pardoned? Then pardon others. Forgive and you will be forgiven. Do you want to receive? Give and you will receive.
If we think of our sins, reckoning up those we have committed by sight, hearing, thought, and countless disorderly emotions, I do not know whether we can even sleep without falling into debt. And so, every day we pray; every day we beat upon God’s ears with our pleas; every day we prostrate ourselves before him saying: Forgive us our trespasses, as we also forgive those who trespass against us. Which of our trespasses, all of them or only some? All, you will answer. Do likewise, therefore, with those who have offended you.
This is the rule you have laid down for yourself, the condition you have stipulated. When you pray according to this pact and covenant you remember to say: Forgive us, as we also forgive our debtors.
St Augustine of Hippo