7 But if you incline to understand it of the house itself, where the feet of that house have stood; let your feet stand in Christ. They will then stand, if you shall persevere in Christ. For what is said of the devil? “He was a murderer from the beginning, and stood not in the truth.” The feet of the devil therefore stood not. Also what says he of the proud? “O let not the foot of pride come against me; and let not the hand of the ungodly cast me down. There are they fallen, all that work wickedness: they are cast down, and were not able to stand.” That then is the house of God, whose feet stand. Whence John rejoicing, says: what? “He that has the bride is the bridegroom: but the friend of the bridegroom stands and hears him.” If he stand not, he hears him not. Justly he stands, because “he rejoices on account of the bridegroom's voice.” Now therefore ye see why they fell, who rejoice because of their own voice. That friend of the Bridegroom said, “The same is He which baptizes.” Some say, We baptize: rejoicing in their own voice, they could not stand; and belong not to that house of which it is said, “where His feet stood.”
8. “Arise, O Lord, into Your resting place”. He says unto the Lord sleeping, “Arise.” You know already who slept, and who rose again....“You, and the ark of Your sanctification:” that is, Arise, that the ark of Your sanctification, which You have sanctified, may arise also. He is our Head; His ark is His Church: He arose first, the Church will arise also. The body would not dare to promise itself resurrection, save the Head arose first. The Body of Christ, that was born of Mary, has been understood by some to be the ark of sanctification; so that the words mean, Arise with Your Body, that they who believe not may handle.
9. “Let Your priests be clothed with righteousness, and let Your saints sing with joyfulness”. When Thou risest from the dead, and go unto Your Father, let that royal Priesthood be clothed with faith, since “the righteous lives by faith;” and, receiving the pledge of the Holy Spirit, let the members rejoice in the hope of resurrection, which went before in the Head: for to them the Apostle says, “Rejoicing in hope.”
10. “For Your servant David's sake, turn not away the face of Your Anointed”. These words are addressed unto God the Father. “For Your servant David's sake, turn not away the face of Your Anointed.” The Lord was crucified in Judæa; He was crucified by the Jews; harassed by them, He slept. He arose to judge those among whose savage hands He slept: and He says elsewhere, “Raise Thou Me up again, and I shall reward them.” He both has rewarded them, and will reward them. The Jews well know themselves how great were their sufferings after the Lord's death. They were all expelled from the very city, where they slew Him. What then? Have all perished even from the root of David and from the tribe of Judah? No: for some of that stock believed, and in fact many thousands of men of that stock believed, and this after the Lord's resurrection. They raged and crucified Him: and afterwards began to see miracles wrought in the Name of Him Crucified; and they trembled still more that His Name should have so much power, since when in their hands He seemed unable to work any; and pricked at heart, at length believing that there was some hidden divinity in Him whom they had believed like other men, and asking counsel of the Apostles, they were answered, “Repent, and be baptized every one of you in the Name of our Lord Jesus Christ.” Since then Christ arose to judge those by whom He had been crucified, and turned away His Presence from the Jews, turning His Presence towards the Gentiles; God is, as it seems, besought in behalf of the remnant of Israel; and it is said unto Him, “For Your servant David's sake, turn not away the presence of Your Anointed.” If the chaff be condemned, let the wheat be gathered together. May the remnant be saved, as Isaiah says, “And the remnant has” clearly “been saved:” for out of them were the twelve Apostles, out of them more than five hundred brethren, to whom the Lord showed Himself after His Resurrection: out of their number were so many thousands baptized, who laid the price of their possessions at the Apostles' feet. Thus then was fulfilled the prayer here made to God: “For Your servant David's sake, turn not away the presence of Your Anointed.”
11. “The Lord has made a faithful oath unto David, and He shall not repent”. What means, “has made an oath”? Hath confirmed a promise through Himself. What means, “He shall not repent”? He will not change. For God suffers not the pain of repentance, nor is He deceived in any matter, so that He would wish to correct that wherein He has erred. But as when a man repents of anything, he wishes to change what he has done; thus where you hear that God repents, look for an actual change. God does it differently from you, although He calls it by the name of repentance; for thou dost it, because you had erred; while He does it, because He avenges, or frees. He changed Saul's kingdom, when He repented, as it is said: and in the very passage where the Scripture says, “It repented Him;” it is said a little after, “for He is not a man that He should repent.” When therefore He changes His works through His immutable counsel, He is said to repent on account of this very change, not of His counsel, but of His work. But He promised this so as not to change it. Just as this passage also says: “The Lord swore, and will not repent, You are a Priest for ever after the order of Melchizedec;” so also since this was promised so that it should not be changed, because it must needs happen and be permanent; he says, “The Lord has made a faithful oath unto David, and He shall not repent; Of the fruit of your body shall I set upon your seat.” He might have said, “of the fruit of your loins,” wherefore did He choose to say, “Of the fruit of your body”? Had He said that also, it would have been true; but He chose to say with a further meaning, Ex fructu ventris, because Christ was born of a woman without the man.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)