8 For we are not more perfect than David, who by a little carelessness was hurled into the very gulf of sin. Yet he arose again quickly. Look not then to his having sinned only, but also to his having washed away his sin. For to this end He wrote that history, not that you should behold him fallen, but admire him risen; to teach you, when you are fallen, how you should arise. Thus, as physicians choose out the most grievous diseases, and write them in their books, and teach their method of cure in similar cases; if so be men having practised on the greater, may easily master the less; even so God likewise has brought forward the greatest of sins, that they also who offend in small things may find the cure of these easy, by means of the other: since if those admitted of healing, much more the less.
Let us look then to the manner both of the sickness, and of the speedy recovery of that blessed man. What then was the manner of his sickness? He committed adultery and murder. For I shrink not from proclaiming these things with a loud voice. Since if the Holy Ghost thought it no shame to record all this history, much less ought we to draw any shade over it. Wherefore I not only proclaim it, but I add another circumstance also. For in fact, whosoever hide these things, they most of all men throw his virtue into the shade. And as they that say nothing of the battle with Goliath deprive him of no small crowns, so also they that hurry by this history. Does not my saying seem a paradox? Nay, wait a little, and then you shall know that with reason have we said this. For to this end do I magnify the sin, and make my statement stranger, that I may the more abundantly provide the medicines.
What is it then which I add? The man's virtue; which makes the fault also greater. For all things are not judged alike in all men. “For mighty” men (it is said) “shall be mightily tormented:” and “He that knew his Lord's will, and does it not, shall be beaten with many stripes.” So that more knowledge is a ground of more punishment. For this same reason the priest, if he commit the same sin as those under government, shall not have the same to endure, but things far more grievous.
Perhaps, seeing the charge against him amplified, you tremble and fear, and marvel at me, as though I were going down a precipice. But I am so confident on that righteous man's behalf, that I will proceed even farther; for the more I aggravate the charge, so much the more shall I be able to show forth the praise of David.
“And what more than this,” you will say, “can be uttered?” Abundantly more. For as in the case of Cain, what was done was not a murder only, but worse than even many murders; for it was not a stranger, but a brother, whom he slew; and a brother who had not done but suffered wrong; not after many murderers, but having first originated the horrid crime: so here too that which was perpetrated was not murder only. For it was no ordinary man that did it, but a prophet: and he slays not him that had done wrong, but him that had suffered wrong; for indeed he had been mortally wronged, by the forcing away his wife: nevertheless after that he added this also.
Source: Homilies on the Gospel of St. Matthew (New Advent)