15 But still you are saying, Who can do, who has ever done this? May God bring it to effect in your hearts! I know as well as you, there are but few who do it; great men are they and spiritual who do so. Are all the faithful in the Church who approach the altar, and take the Body and Blood of Christ, are they all such? And yet they all say, “Forgive us our debts, as we also forgive our debtors.” What, if God should answer them, “Why do ye ask me to do what I have promised, when you do not what I have commanded?”
What have I promised? “To forgive your debts.” What have I commanded? “That ye also forgive your debtors.” How can you do this, if you do not love your enemies? What then must we do, brethren? Is the flock of Christ reduced to such a scanty number? If they only ought to say, “Forgive us our debts, as we also forgive our debtors,” who love their enemies; I know not what to do, I know not what to say. For must I say to you, If you do not love your enemies, do not pray; I dare not say so; yea, pray rather that you may love them.
But must I say to you, If you do not love your enemies, say not in the Lord's Prayer, “Forgive us our debts, as we also forgive our debtors”? Suppose that I were to say, Do not use these words. If you do not, your debts are not forgiven; and if you do use them, and do not act thereafter, they are not forgiven. In order therefore that they may be forgiven, you must both use the prayer, and do thereafter.
Source: Sermons on Selected Lessons of the New Testament (New Advent)