6 But what he says, “My pilgrimage has been made distant,” are the words of those, that is, of the Church herself, who toils on this earth. It is her voice, which cries out from the ends of the earth in another Psalm, saying, From the ends of the earth have I cried unto You....Where then does he groan, and among whom does he dwell? “I have had my habitation among the tents of Kedar.” Since this is a Hebrew word, beyond doubt you have not understood it. What means, “I have had my habitation among the tents of Kedar”? “Kedar,” as far as we remember of the interpretation of Hebrew words, signifies darkness. “Kedar” rendered into Latin is called tenebræ. Now ye know that Abraham had two sons, whom indeed the Apostle mentions, and declares them to have been types of the two covenants...Ishmael therefore was in darkness, Isaac in light. Whoever here also seek earthly felicity in the Church, from God, shall belong to Ishmael. These are the very persons who gainsay the spiritual ones who are progressing, and detract from them, and have deceitful tongues and unrighteous lips. Against these the Psalmist, when ascending, prayed, and hot coals that lay waste, and swift and sharp arrows of the Mighty One, were given him for his defence. For among these he still lives, until the whole floor be winnowed: he therefore said, “I have dwelt among the tents of Kedar.” The tents of Ishmael are called those of Kedar. Thus the book of Genesis has it: thus it has, that Kedar belongs unto Ishmael. Isaac therefore is with Ishmael: that is, they who belong unto Isaac, live among those who belong unto Ishmael. These wish to rise above, those wish to press them downwards: these wish to fly unto God, those endeavour to pluck their wings...
7. “My soul has wandered much”. Lest you should understand bodily wandering, he has said that the soul wandered. The body wanders in places, the soul wanders in its affections. If you love the earth, you wander from God: if you love God, you rise unto God. Let us be exercised in the love of God, and of our neighbour, that we may return unto charity. If we fall towards the earth, we wither and decay. But one descended unto this one who had fallen, in order that he might arise. Speaking of the time of his wandering, he said that he wandered in the tents of Kedar. Wherefore? Because “my soul has wandered much.” He wanders there where he ascends. He wanders not in the body, he rises not in the body. But wherein does he ascend? “The ascent,” he says, “is in the heart.”
8. “With them that hated peace, I was peaceful”. But howsoever ye may hear, most beloved brethren, you will not be able to prove how truly ye sing, unless you have begun to do that which you sing. How much soever I say this, in whatsoever ways I may expound it, in whatsoever words I may turn it, it enters not into the heart of him in whom its operation is not. Begin to act, and see what we speak. Then tears flow forth at each word, then the Psalm is sung, and the heart does what is sung in the Psalm...Who are they who hate peace? They who tear asunder unity. For had they not hated peace, they would have abode in unity. But they separated themselves, forsooth on this account, that they might be righteous, that they might not have the ungodly mixed with them. These words are either ours or theirs: decide whose. The Catholic Church says, Unity must not be lost, the Church of God must not be cut off. God will judge afterwards of the wicked and the good...This we also say: Love ye peace, love ye Christ. For if they love peace, they love Christ. When therefore we say, Love ye peace, we say this, Love ye Christ. Wherefore? For the Apostle says of Christ, “He is our peace, who has made both one.” If Christ is therefore peace, because He has made both one: why have ye made two of one? How then are you peace-makers, if, when Christ makes one of two, you make two of one? But since we say these things, we are peace-makers with them that hate peace; and yet they who hate peace, when we spoke to them, made war on us for nought.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)