28 The fifth petition follows: “And forgive us our debts, as we also forgive our debtors.” It is manifest that by debts are meant sins, either from that statement which the Lord Himself makes, “You shall by no means come out thence, till you have paid the uttermost farthing;” or from the fact that He called those men debtors who were reported to Him as having been killed, either those on whom the tower fell, or those whose blood Herod had mingled with the sacrifice. For He said that men supposed it was because they were debtors above measure, i.e. sinners, and added “I tell you, Nay: but, unless you repent, you shall all likewise die.” Here, therefore, it is not a money claim that one is pressed to remit, but whatever sins another may have committed against him. For we are enjoined to remit a money claim by that precept rather which has been given above, “If any man will sue you at the law, and take away your coat, let him have your cloak also;” nor is it necessary to remit a debt to every money debtor; but only to him who is unwilling to pay, to such an extent that he wishes even to go to law. “Now the servant of the Lord,” as says the apostle, “must not go to law.” And therefore to him who shall be unwilling, either spontaneously or when requested, to pay the money which he owes, it is to be remitted. For his unwillingness to pay will arise from one of two causes, either that he has it not, or that he is avaricious and covetous of the property of another; and both of these belong to a state of poverty: for the former is poverty of substance, the latter poverty of disposition. Whoever, therefore, remits a debt to such an one, remits it to one who is poor, and performs a Christian work; while that rule remains in force, that he should be prepared in mind to lose what is owing to him. For if he has used exertion in every way, quietly and gently, to have it restored to him, not so much aiming at a money profit, as that he may bring the man round to what is right, to whom without doubt it is hurtful to have the means of paying, and yet not to pay; not only will he not sin, but he will even do a very great service, in trying to prevent that other, who is wishing to make gain of another's money, from making shipwreck of the faith; which is so much more serious a thing, that there is no comparison. And hence it is understood that in this fifth petition also, where we say, “Forgive us our debts,” the words are spoken not indeed in reference to money, but in reference to all ways in which any one sins against us, and by consequence in reference to money also. For the man who refuses to pay you the money which he owes, when he has the means of doing so, sins against you. And if you do not forgive this sin, you will not be able to say, “Forgive us, as we also forgive;” but if you pardon it, you see how he who is enjoined to offer such a prayer is admonished also with respect to forgiving a money debt.
29. That may indeed be construed in this way, that when we say, “Forgive us our debts, as we also forgive,” then only are we convicted of having acted contrary to this rule, if we do not forgive them who ask pardon, because we also wish to be forgiven by our most gracious Father when we ask His pardon. But, on the other hand, by that precept whereby we are enjoined to pray for our enemies, it is not for those who ask pardon that we are enjoined to pray. For those who are already in such a state of mind are no longer enemies. By no possibility, however, could one truthfully say that he prays for one whom he has not pardoned. And therefore we must confess that all sins which are committed against us are to be forgiven, if we wish those to be forgiven by our Father which we commit against Him. For the subject of revenge has been sufficiently discussed already, as I think.
Source: Our Lord's Sermon on the Mount (New Advent)