4 What therefore is, “When he fled from the face of Saul into a cavern”? Which thing indeed the former David also did: but because in him we find not the inscription of the title, in the latter let us find the flight into the cavern. For that cavern wherein David hid himself did figure somewhat. But wherefore hid he himself? It was in order that he might be concealed and not be found. What is to be hidden in a cavern? To be hidden in earth. For he that flees into a cavern, with earth is covered so that he may not be seen. But Jesus did carry earth, flesh which He had received from earth: and in it He concealed Himself, in order that by Jews He might not be discovered as God. “For if they had known, never the Lord of glory would they have crucified.” Why therefore the Lord of glory found they not? Because in a cavern He had hidden Himself, that is, the flesh's weakness to their eyes He presented, but the Majesty of the Godhead in the body's clothing, as though in a hiding-place of the earth, He hid....But wherefore even unto death willed He to be patient? It was in order that He might flee from the face of Saul into a cavern. For a cavern may be understood as a lower part of the earth. And certainly, as is manifest and certain to all, His Body in a Tomb was laid, which was cut in a Rock. This Tomb therefore was the Cavern; there He fled from the face of Saul. For so long the Jews did persecute Him, even until He was laid in a cavern. Whence prove we that so long they persecuted Him, until therein He was laid? Even when dead, and, on the Cross hanging, with lance they wounded Him. But when shrouded, the funeral celebrated, He was laid in a cavern, no longer had they anything which to the Flesh they might do. Rose therefore the Lord again out of that cavern unhurt, uncorrupt, from that place whither He had fled from the face of Saul: concealing Himself from ungodly men, whom Saul prefigured, but showing Himself to His members. For the members of Him rising again by His members were handled: for the members of Him, the Apostles, touched Him rising again and believed; and behold nothing profited the persecution of Saul. Hear we therefore now the Psalm; because concerning the title thereof enough we have spoken, as far as the Lord has deigned to give.
5. “Have pity on me, O God, have pity on me, for in You has trusted my Soul”. Christ in the Passion says, “Have pity on Me, O God.” To God, God says, “Have pity on Me!” He that with the Father has pity on you, in you cries, “Have pity on Me.” For that part of Him which is crying, “Have pity on Me,” is yours: from you this He received, for the sake of you, that you should be delivered, with Flesh He was clothed. The flesh itself cries: “Have pity on Me, O God, have pity on me:” Man himself, soul and flesh. For whole Man did the Word take upon Him, and whole Man the Word became. Let it not therefore be thought that there Soul was not, because the Evangelist thus says: “The Word was made flesh, and dwelled in us.” For man is called flesh, as in another place says the Scripture, “And all flesh shall see the salvation of God.” Shall anywise flesh alone see, and shall Soul not be there?...You hear the Master praying, learn thou to pray. For to this end He prayed, in order that He might teach how to pray: because to this end He suffered, in order that He might teach how to suffer; to this end He rose again, in order that He might teach how to hope for rising again. “And in the shadow of Your wings I will hope, until iniquity pass over.” This now evidently whole Christ does say: here is also our voice. For not yet has passed over, still rife is iniquity. And in the end our Lord Himself said there should be an abounding of iniquity: “And since iniquity shall abound, the love of many shall wax cold; but he that shall have persevered unto the end, the same shall be saved.” But who shall persevere even unto the end, even until iniquity pass over? He that shall have been in the Body of Christ, he that shall have been in the members of Christ, and from the Head shall have learned the patience of persevering. Thou passest away, and behold passed are your temptations; and you go into another life whither have gone holy men, if holy you have been. Into another life have gone Martyrs; if Martyr you shall have been, you also go into another life. Because “you” have passed away hence, has by any means iniquity therefore passed away? There are born other unrighteous men, as there die some unrighteous men. In like manner therefore as some unrighteous men die and others are born: so some just men go, and others are born. Even unto the end of the world neither iniquity will be wanting to oppress, nor righteousness to suffer....
6. “I will cry to God most high”. If most high He is, how hears He you crying? Confidence has been engendered by experience: “to God,” he says, “who had done good to me.” If before that I was seeking Him, He did good to me, when I cry shall He not hearken to me? For good to us the Lord God has done in sending to us our Saviour Jesus Christ, that He might die for our offenses, and rise again for our justification. For what sort of men has He willed His Son to die? For ungodly men. But ungodly men were not seeking God, and have been sought of God. For He is Most High in such sort, as that not far from Him is our misery and our groaning: because “near is the Lord to them that have bruised the heart.” “God that has done good to me.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)