Psalms 141:3

But what is true is also the healthy confession of sins. Hence we heard in the psalm that was read first: Set a guard, O Lord, over my mouth; keep watch over the door of my lips, so that my heart may not incline to evil words, to make excuses for sins. He asks God to set a guard over his mouth and explains what kind of guard he means. For there are people who are very abundant, who, when they begin to be accused, rush to excuse themselves. To excuse is to seek causes and to weave fabrications so that the sin does not seem to pertain to you. One says: "The devil made me do it"; another says: "Fortune made me do it"; another: "I was compelled by fate," no one to himself. When you wish to be your own excuser, your accuser triumphs over you. Do you wish to do what will cause your accuser, that is, the devil, to grieve and lament? Do what you heard, do what you learned, and say to your God: I said, O Lord, have mercy on me; heal my soul, for I have sinned against you. I, he says, I have said, not the devil, not fortune, not fate. I said: I do not excuse myself, but I accuse myself. I said, have mercy on me, heal my soul. Why is it sick? Because I have sinned against you.

And lest it should seem that necessary Continence was to be hoped for from the Lord only in respect of the lust of the lower parts of the flesh, it is also sung in the Psalm; "Set, O Lord, a watch to my mouth, and a door of Continence around my lips." But in this witness of the divine speech, if we understand "mouth" as we ought to understand it, we perceive how great a gift of God Continence there set is. Forsooth it is little to contain the mouth of the body, lest any thing burst forth thence, which is not for the better, through the sound of the voice. For there is, within, the mouth of the heart, where he, who spake these words, and wrote them for us to speak, desired of the Lord that the watch and door of Continence should be set for him. For many things we say not with the mouth of the body, and cry aloud with the heart: but there goes forth from the mouth of the body no word of any thing, whereof there is silence in the heart. Therefore what flows not forth thence, sounds not abroad: but what flows forth thence, if it be evil, although it move not the tongue, defiles the soul. Therefore Continence must be set there, where the conscience even of them who are silent speaks. For it is brought to pass by means of the door of Continence, that there go not forth thence that, which, even when the lips of the flesh are closed, pollutes the life of him that hath the thought. Lastly, to show more plainly the inner mouth, which by these words he meant, after having said, "Set a watch, O Lord, to my mouth, and a door of Continence around my lips," he added straightway, "Cause not my heart to fall aside into evil words." The failing aside of the heart, what is it but the consent? For he hath not yet spoken, whosoever in his heart hath with no failing aside of the heart consented unto suggestions that meet him of each several thing that is seen. […]

Above all, indeed, we must ensure that we do not sin, nor form any familiarity and friendship with sin as with a serpent. For it kills the sinner with a venomous bite, and there is nothing with which friendship is to be made. But if by chance it oppresses the weak or deceives the unsuspecting or captures the erring or ensnares them in repeated error, let the soul not hesitate to confess and not seek excuse but self-accusation. For this reason, one also prayed in a psalm and said: "Set a guard, O Lord, over my mouth; keep watch over the door of my lips. Incline not my heart to any evil, to busy myself in wicked deeds in company with men who work iniquity." Does sin entice you? Before all else, reject it. But if persuaded, do not excuse it but rather accuse it.

But the blow inflicted by the tongue is incurable. The tongue strikes lightly, but it always stirs up deep sighs in the chest through the sorrow it causes. The prophet no doubt knew how great was the evil of the tongue when he cried out, …

..."Set, O Lord, a watch before my mouth, and a door of restraint around my lips" (ver. 3). He said not a barrier of restraint, but "a door of restraint." A door is opened as well as shut. If then it be a "door," let it be both opened and shut; opened, to confession of sin; closed, to excusing sin. So will it be a "door of restraint," not of ruin. For what doth this "door of restraint" profit us? What doth Christ pray in the name of His Body? "That Thou turn not aside My heart to wicked words" (ver. 4). What is, "My heart"? The heart of My Church; the heart, that is, of My Body. ...