5 I would then that you should forgive, seeing that I find you asking forgiveness. You are asked, forgive: you are asked, and you will ask yourself; you are asked, forgive; you will ask to be forgiven; for, lo, the time of prayer will come: I have you fast in the words you will have to speak. You will say, “Our Father, which art in heaven.” For you will not be in the number of children, if you shall not say, “Our Father.” So then you will say, “Our Father, which art in heaven.”
Follow on; “Hallowed be Your Name.” Say on, “Your kingdom come.” Follow still on, “Your will be done, as in heaven, so in earth.” See what you add next, “Give us this day our daily bread.” Where are your riches? So you are a beggar. Nevertheless in the mean while (it is the point I am speaking of), say what is next after, “Give us this day our daily bread.” Say what follows this: “Forgive us our debts.” Now you have come to my words, “Forgive us our debts.” By what right? By what covenant?
On what condition? On what express stipulation? “As we also forgive our debtors.” It is but a small thing that you do not forgive; yea you do more, you lie unto God. The condition is laid down, the law fixed. “Forgive as I forgive.” Therefore He does not forgive, unless you forgive. “Forgive as I forgive.” You wish to be forgiven when you ask; forgive him that asks of you. He that is skilled in heaven's laws has dictated these prayers: He does not deceive you; ask according to the tenor of His heavenly voice: say, “Forgive us, as we also forgive,” and do what you say.
He that lies in his prayers, loses the benefit he seeks: he that lies in his prayers, both loses his cause, and finds his punishment. And if any one lies to the emperor, he is convicted of his lie at his coming: but when you lie in prayer, you by your very prayer are convicted. For God does not seek for witness as regards you to convict you. He who dictated the prayers to you, is your Advocate: if you lie, He is a witness against you: if you do not amend yourself, He will be your Judge.
So then both say it, and do. For if you say it not, you will not obtain making your requests contrary to the law; but if you say it and do it not, you will be further guilty of lying. There is no means of evading that verse, save by fulfilling what we say. Can we blot this verse out of our prayer? Would ye that clause, “Forgive us our debts,” should be there, and that we should blot out what follows, “As we also forgive our debtors”? You shall not blot it out, lest you be first blotted out yourself.
So then in this prayer you say, “Give,” and you say, “Forgive:” that you may receive what you have not, and may be forgiven what you have done amiss. So then you wish to receive, give; you wish to be forgiven, forgive. It is a brief summary. Hear Christ Himself in another place, “Forgive, and you shall be forgiven.” What will you forgive? What others have sinned against you. What shall you be forgiven? What you have sinned yourselves. “Forgive.” “Give, and there shall be given you what ye desire,” eternal life. Support the temporal life of the poor man, sustain the poor man's present life, and for this so small and earthly seed you shall receive for harvest life eternal. Amen.
Source: Sermons on Selected Lessons of the New Testament (New Advent)