2 Therefore, down from the higher place fortified and protected, he, to whom the Lord has been made a refuge, he, to whom is God Himself for a fortified place, has regard to those whom he has leaped over, and looking down upon them speaks as though from a lofty tower: for this also has been said of Him, “A Tower of strength from the face of the enemy:” he gives heed therefore to them, and says, “How long do ye lay upon a man?”. By insulting, by hurling reproaches, by laying wait, by persecuting, you lay upon a man burthens, you lay upon a man as much as a man can bear: but in order that a man may bear, under him is He that has made man.
If to a man ye look, “slay ye, all of you.” Behold, lay upon, rage, “slay ye, all of you.” “As though a wall bowed down, and as a fence smitten against;” lean against, smite against, as if going to throw down. And where is, “I shall not be moved more”? But wherefore? “I shall not be moved more.” Because Himself is God my Saving One, my taker up, therefore ye men are able to lay burdens upon a man; can you anywise lay upon God, who protects man? “Slay ye, all of you.” What is that size of body in one man so great as that he may be slain by all?
But we ought to perceive our person, the person of the Church, the person of the Body of Christ. For one Man with His Head and Body is Jesus Christ, the Saviour of the Body and the Members of the Body: two in one Flesh, and in one voice, and in one passion, and, when iniquity shall have passed over, in one rest. The sufferings therefore of Christ are not in Christ alone; nay, there are not any save in Christ. For if Christ you understand to be Head and Body, the sufferings of Christ are not, save in Christ: but if Christ thou understand of Head alone, the sufferings of Christ are not in Christ alone.
For if the sufferings of Christ are in Christ alone, to wit in the Head alone; whence says a certain member of Him, Paul the Apostle, “In order that I may supply what are wanting of the oppressions of Christ in my flesh”? If therefore in the members of Christ you are, whatsoever man you are that art hearing these words, whosoever you are that dost hear these words (but however, you hear, if in the members of Christ you are): whatsoever thing you suffer from those that are not in the members of Christ, was wanting to the sufferings of Christ.
Therefore it is added because it was wanting; you fill up the measure, you cause it not to run over: you suffer so much as was to be contributed out of your sufferings to the whole suffering of Christ, that has suffered in our Head, and does suffer in His members, that is, in our own selves. Unto this our common republic, as it were each of us according to our measure pays that which we owe, and according to the powers which we have, as it were a quota of sufferings we contribute.
The storehouse of all men's sufferings will not be completely made up, save when the world shall have been ended....That whole City therefore is speaking, from the blood of righteous Abel even to the blood of Zacharias. Thence also hereafter from the blood of John, through the blood of the Apostles, through the blood of Martyrs, through the blood of the faithful ones of Christ, one City speaks, one man says, “How long do ye lay upon a man? Slay ye, all of you.” Let us see if you efface, let us see if you extinguish, let us see if you remove from the earth the name thereof, let us see if you peoples do not meditate of empty things, saying, “When shall She die, and when shall perish the name of Her?” “As though She were a wall bowed down, and a fence smitten against,” lean ye against Her, smite against Her.
Hear from above: “My taker up, I shall not be moved more:” for as though a heap of sand I have been smitten against that I might fall, and the Lord has taken me up.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)