11 And truly it is a great temptation, dearly beloved, it is a great temptation in this life, when that in us is the subject of temptation, whereby we attain pardon, if in any of our temptations we have fallen. It is a frightful temptation, when that is taken from us, whereby we may be healed from the wounds of other temptations. I know that you have not yet understood me. Give me your attention, that you may understand. Suppose avarice tempts a man, and he is conquered in any single temptation (for sometimes even a good wrestler and fighter may get roughly handled): avarice then has got the better of a man, good wrestler though he be, and he has done some avaricious act.
Or there has been a passing lust; it has not brought the man to fornication, nor reached unto adultery, for when this does take place, the man must at all events be kept back from the criminal act. But he “has seen a woman to lust after her;” he has let his thoughts dwell on her with more pleasure than was right; he has admitted the attack; excellent combatant though he be, he has been wounded, but he has not consented to it; he has beaten back the motion of his lust, has chastised it with the bitterness of grief, he has beaten it back; and has prevailed.
Still in the very fact that he had slipped, has he ground for saying, “Forgive us our debts.” And so of all other temptations, it is a hard matter that in them all there should not be occasion for saying, “Forgive us our debts.” What then is that frightful temptation which I have mentioned, that grievous, that tremendous temptation, which must be avoided with all our strength, with all our resolution; what is it? When we go about to avenge ourselves. Anger is kindled, and the man burns to be avenged.
O frightful temptation! You are losing that, whereby you had to attain pardon for other faults. If you had committed any sin as to other senses, and other lusts, hence might you have had your cure, in that you might say, “Forgive us our debts, as we also forgive our debtors.” But whoso instigates you to take vengeance, will lose for you the power you had to say, “As we also forgive our debtors.” When that power is lost, all sins will be retained; nothing at all is remitted.
Source: Sermons on Selected Lessons of the New Testament (New Advent)