1....Our Lord Jesus Christ speaks in the Prophets, sometimes in His own Name, sometimes in ours, because He makes Himself one with us; as it is said, “they two shall be one flesh.” Wherefore also the Lord says in the Gospel, speaking of marriage, “therefore they are no more two, but one flesh.” One flesh, because of our mortality He took flesh; not one divinity, for He is the Creator, we the creature. Whatsoever then our Lord speaks in the person of the Flesh He took upon Him, belongs both to that Head which has already ascended into heaven, and to those members which still toil in their earthly wandering. Let us hear then our Lord Jesus Christ speaking in prophecy. For the Psalms were sung long before the Lord was born of Mary, yet not before He was Lord: for from everlasting He was the Creator of all things, but in time He was born of His creature. Let us believe that Godhead, and, so far as we can, understand Him to be equal to the Father. But that Godhead equal to the Father was made partaker of our mortal nature, not of His own store, but of ours; that we too might be made partakers of His Divine Nature, not of our store, but of His.
2. “Lord, You have tried me, and known me”. Let the Lord Jesus Christ Himself say this; let Him too say, “Lord,” to the Father. For His Father is not His Lord, save because He has deigned to be born according to the flesh. He is Father of the God, Lord of the Man. Would you know to whom He is Father? To the coequal Son. The Apostle says, “Who, being in the form of God, thought it not robbery to be equal with God.” To this “Form” God is Father, the “Form” equal to Himself, the only-begotten Son, begotten of His Substance. But forasmuch as for our sakes, that we might be re-made, and made partakers of His Divine Nature, being renewed unto life eternal, He was made partaker of our mortal nature, what says the Apostle of Him? He says, “yet He emptied Himself, and took upon Him the form of a servant, and was made in the likeness of men, and was found in fashion as a man.” He was in the Form of God, equal to the Father; He took upon Him the form of a servant, so as therein to be less than the Father....
3. “You have known My down-sitting and Mine up-rising”. What here is “down-sitting,” what “up-rising”? He who sits, humbles himself. The Lord then “sat” in His Passion, “up-rose” in His Resurrection. “You,” he says, hast known this; that is, You have willed, You have approved; according to Your will was it done. But if you choose to take the words of the Head in the person of the Body: man sits when he humbles himself in penitence, he rises up when his sins are forgiven, and he is lifted up to the hope of everlasting life. Lift not up yourselves, unless you have first been humbled. For many wish to rise before they have sat down, they wish to appear righteous, before they have confessed that they are sinners....
4. “You have understood my thoughts from afar; You have tracked out my path and my limit”; “and all my ways You have seen beforehand”. What is, “from afar”? While I am yet in my pilgrimage, before I reach that, my true country, You have known my thoughts....The younger son went into a far country. After his toil and suffering and tribulation and want, he thought on his father, and desired to return, and said, “I will arise, and go to my father.” “I will arise,” said he, for before he had sat. Here then you may recognise him saying, “You have known my down-sitting and up-rising.” I sat, in want; I arose, in longing for Your Bread. “You have understood my thoughts from afar.” For far indeed had I gone; but where is not He whom I had left? Wherefore the Lord says in the Gospel, that his father met him as he was coming. Truly; for “he had understood his thoughts from afar.” “My path,” he says; what, but a bad path, the path he had walked to leave his father?...What is, “my path”? That by which I have gone. What is, “my limit”? That whereunto I have reached. “You have tracked out my path and my limit.” That limit of mine, far distant as it was, was not far from Your eyes. Far had I gone, and yet You were there. “And all my ways You have seen beforehand.” He said not, “hast seen,” but, “hast seen beforehand.” Before I went by them, before I walked in them, You saw them beforehand; and You permitted me in toil to go my own ways, that, if I desired not to toil, I might return into Your ways. “For there is no deceit in my tongue.” What meant he by this? Lo, I confess to You, I have walked in my own way, I have become far from You, I have departed from You, with whom it was well with me, and to my good it was ill with me without You....
5. “Behold Thou, Lord, hast known all my last doings, and the ancient ones”. You have known my latest doings, when I fed swine; You have known my ancient doings, when I asked of You my portion of goods. Ancient doings were the beginnings to me of latest ills: ancient sin, when we fell; latest punishment, when we came into this toilsome and dangerous mortality. And would that this may be “latest” to us; it will be, if now we will to return. For there is another “latest” for certain wicked ones, to whom it shall be said, “Go ye into everlasting fire.”...“You have fashioned me, and hast laid Your hand upon me.” “Fashioned me,” where? In this mortality; now, to the toils whereunto we all are born. For none is born, but God has fashioned him in his mother's womb; nor is there any creature, whereof God is not the Fashioner. But “You have fashioned me” in this toil, “and laid Your hand upon me,” Your avenging hand, putting down the proud. For thus healthfully has He cast down the proud, that He may lift him up humble.
6. “Your skill has displayed itself wonderfully in me: it has waxed mighty: I shall not be able to attain unto it”. Listen now and hear somewhat, which is obscure indeed, yet brings no small pleasure in the understanding thereon. Moses, the holy servant of God, with whom God spoke by a cloud, for, speaking after human fashion, He must needs speak to His servant through some work of His hands which He assumed,...longed and desired to see the true appearance of God, and said to God, who was conversing with him, “If now I have found grace in Your sight, show me Yourself.” When this he desired vehemently, and would extort from God in that sort of friendly familiarity, if we may so speak, wherewith God deigned to treat him, that he might see His Glory and His Face, in such wise as we can speak of God's Face, He said unto him, “You can not see My Face; for no one has seen My Face, and lived;” but I will place you in a cleft of the rock, and will pass by, and will set My hand upon you; and when I have passed by, you shall see My back parts. And from these words there arises another enigma, that is, an obscure figure of the truth. “When I have passed by,” says God, “you shall see My back parts;” as though He has on one side His face, on another His back. Far be it from us to have any such thoughts of that Majesty! For whoso has such thoughts of God, what advantages it him that the temples are closed? He is building an idol in his own heart. In these words then are mighty mysteries....They who raged against the Lord, whom they saw, now seek counsel how they may be saved; and it is said to them, “Repent, and be baptized every one of you in the Name of Jesus Christ, and your sins shall be forgiven you.” Behold, they saw the back parts of Him, whose face they could not see. For His Hand was upon their eyes, not for ever, but while He passed by. After He had passed He took away His Hand from their eyes. When the hand was taken from their eyes, they say to the disciples, “What shall we do?” At first they are fierce, afterwards loving; at first angry, afterwards fearful; at first hard, then pleasant; at first blind, then enlightened....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)