3 Perhaps we should enquire respecting this word also, why he said, “He is girded.” Girding signifies work: for every man then girds himself, when he is about to work. But wherefore did he use the word præcinctus, instead of cinctus? For he says in another Psalm, “Gird You with Your sword upon Your thigh, O Thou most mighty: the people shall fall under You:” using the word accingere, not cingere, nor præcingere: this word being applied to the act of attaching anything to the side by girding it. The sword of the Lord, wherewith He conquered the round world by killing iniquity, is the Spirit of God in the truth of the word of God. Wherefore is He said to bind His sword around His thigh? In another place, on another Psalm we have spoken in another manner of girding: but nevertheless, since it has been mentioned, it ought not to be passed over. What is the girding on of the sword around the thigh? He means the flesh by the thigh. For the Lord would not otherwise conquer the round world, unless the sword of truth came into the flesh. Why therefore is He here said to be girded in front (præcinctus)? He who girds himself before, places something before himself, wherewith he is girded; whence it is said, He girded Himself before with a towel, and began to wash the disciples' feet. Because He was humble when He girded Himself with a towel. He washed the feet of His own disciples. But all strength is in humility: because all pride is fragile: therefore when He was speaking of strength, he added, “He is girded:” that you may remember how your God was girded in humility, when He washed His disciples' feet....After He had washed their feet, again He sat down; He said unto them, “You call me Lord and Master: and you say well; for so I am. If I then, your Lord and Master, have washed your feet; how ought ye also to do to one another's feet?” If therefore strength is in humility, fear not the proud. The humble are like a rock: the rock seems to lie downwards: but nevertheless it is firm. What are the proud? Like smoke: although they are lofty, they vanish. We ought therefore to ascribe our Lord's being girded to His humility, according to the mention of the Gospel, that He was girded, that He might wash His disciples' feet.
4....“For He has made the round world sure, which cannot be moved.”...What then is the round world, “which cannot be moved”? This He would not mention specially, if there were not also a round world that can be moved. There is a round world that shall not be moved. There is a round world that shall be moved. For the good who are steadfast in the faith are the round world: that no man may say, they are only in part of it; while the wicked who abide not in faith, when they have felt any tribulation, are throughout the whole world. There is therefore a round world movable: there is a world immovable: of which the Apostle speaks. Behold, the round world movable. I ask you, of whom speaks the Apostle in these words, “Of whom is Hymenæus and Philetus; who concerning the truth have erred, saying that the resurrection is past already: and overthrow the faith of some?” Did these belong to the round world, that shall not be moved? But they were chaff: and as he says, “they overthrow the faith of some.”...“Nevertheless, the foundation of God stands sure; having his seal,”— what seal has it as its sure foundation?— “The Lord knows them that are His.” This is the round world that shall not be moved; “The Lord knows them that are His.” And what seal has it? “And let every one that names the name of Christ depart from unrighteousness.” Let him depart from unrighteousness: for he cannot depart from the unrighteous, for the chaff is mixed with the wheat until it is fanned....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)