For it usually comes to pass with the patient that at the time, indeed, when they suffer hardships, or hear insults, they are smitten with no vexation, and so exhibit patience as to fail not to keep also innocence of heart; but, when after a while they recall to memory these very same things that they have endured, they inflame themselves with the fire of vexation, they seek reasons for vengeance, and, in retracting, turn into malice the meekness which they had in bearing. Such are the sooner succoured by the preacher, if the cause of this change be disclosed.
For the cunning adversary wages war against two; that is, by inflaming one to be the first to offer insults, and provoking the other to return insults under a sense of injury. But for the most part, while he is already conqueror of him who has been persuaded to inflict the injury, he is conquered by him who bears the infliction with an equal mind. Wherefore, being victorious over the one whom he has subjugated by incensing him, he lifts himself with all his might against the other, and is grieved at his firmly resisting and conquering; and so, because he has been unable to move him in the very flinging of insults, he rests meanwhile from open contest, and provoking his thought by secret suggestion, seeks a fit time for deceiving him.
For, having lost in public warfare, he burns to lay hidden snares. In a time of quiet he returns to the mind of the conqueror, brings back to his memory either temporal harms or darts of insults, and by exceedingly exaggerating all that has been inflicted on him represents it as intolerable: and with so great vexation does he perturb the mind that for the most part the patient one, led captive after victory, blushes for having borne such things calmly, and is sorry that he did not return insults, and seeks to pay back something worse, should opportunity be afforded.
To whom, then, are these like but to those who by bravery are victorious in the field, but by negligence are afterwards taken within the gates of the city? To whom are they like but to those whom a violent attack of sickness removes not from life, but who die from a relapse of fever coming gently on? Therefore the patient are to be admonished, that they guard their heart after victory; that they be on the lookout for the enemy, overcome in open warfare, laying snares against the walls of their mind; that they be the more afraid of a sickness creeping on again; lest the cunning enemy, should he afterwards deceive them, rejoice with the greater exultation in that he treads on the necks of conquerors which had long been inflexible against him.
Source: Pastoral Rule (New Advent)