The Enchiridion by St. Augustine
Chapter 73.--The Greatest of All Alms is to Forgive Our Debtors and to Love Our Enemies.
But none of those is greater than to forgive from the heart a sin that has been committed against us. For it is a comparatively small thing to wish well to, or even to do good to, a man who has done no evil to you. It is a much higher thing, and is the result of the most exalted goodness, to love your enemy, and always to wish well to, and when you have the opportunity, to do good to, the man who wishes you ill, and, when he can, does you harm. This is to obey the command of God: |Love your enemies, do good to them that hate you, and pray for them which persecute you.| But seeing that this is a frame of mind only reached by the perfect sons of God, and that though every believer ought to strive after it, and by prayer to God and earnest struggling with himself endeavor to bring his soul up to this standard, yet a degree of goodness so high can hardly belong to so great a multitude as we believe are heard when they use this petition, |Forgive us our debts, as we forgive our debtors;| in view of all this, it cannot be doubted that the implied undertaking is fulfilled if a man, though he has not yet attained to loving his enemy, yet, when asked by one who has sinned against him to forgive him his sin, does forgive him from his heart. For he certainly desires to be himself forgiven when he prays, |as we forgive our debtors,| that is, Forgive us our debts when we beg forgiveness, as we forgive our debtors when they beg forgiveness from us.