5 “Be Thou unto me for a protecting God”. Let not the darts of the enemy reach unto me: for I am not able to protect myself. And a small thing is “protecting:” he has added, “and for a walled place, that You may save me.” “For a walled place” be Thou to me, be Thou my walled place....Behold, God Himself has become the place of your fleeing unto, who at first was the fearful object of your fleeing from. “For a walled place,” he says, be to me, “that You may save me.” I shall not be safe except in You: except You shall have been my rest, my sickness shall not be able to be made whole. Lift me from the earth; upon You I will lie, in order that I may rise unto a walled place. What can be better walled? When unto that place you shall have fled for refuge, tell me what adversaries you will dread? Who will lie in wait, and come at you? A certain man is said from the summit of a mountain to have cried out, when an Emperor was passing by, “I speak not of you:” the other is said to have looked back and to have said, “Nor I of you.” He had despised an Emperor with glittering arms, with mighty army. From whence? From a strong place. If he was secure on a high spot of earth, how secure are you on Him by whom heaven and earth were made? I, if for myself I shall have chosen another place, shall not be able to be safe. Choose thou indeed, O man, if you shall have found one, a place better walled. There is not then a place whither to flee from Him, except we flee to Him. If you will escape Him angry, flee to Him appeased. “For my firmament and my refuge You are.” “My firmament” is what? Through You I am firm, and by You I am firm. “For my firmament and my refuge You are:” in order that I may be made firm by You, in whatever respects I shall have been made infirm in myself, I will flee for refuge unto You. For firm the grace of Christ makes you, and immovable against all temptations of the enemy. But there is there too human frailness, there is there still the first captivity, there is there too the law in the members fighting against the law of the mind, and willing to lead captive in the law of sin: still the body which is corrupt presses down the soul. Howsoever firm thou be by the grace of God, so long as thou still bearest an earthly vessel, wherein the treasure of God is, something must be dreaded even from that same vessel of clay. Therefore “my firmament You are,” in order that I may be firm in this world against all temptations. But if many they are, and they trouble me: “my refuge You are.” For I will confess mine infirmity, to the end that I may be timid like a “hare,” because I am full of thorns like a “hedgehog.” And as in another Psalm is said, “The rock is a refuge for the hedgehogs and the hares:” but the Rock was Christ.
6. “O God, deliver me from the hand of the sinner”. Generally, sinners, among whom is toiling he that is now to be delivered from captivity: he that now cries, “Unhappy man I, who shall deliver me from the body of this death? The grace of God through Jesus Christ our Lord.” Within is a foe, that law in the members; there are without also enemies: unto what do you cry? Unto Him, to whom has been cried, “From my secret sins cleanse me, O Lord, and from strange sins spare Your servant.”...But these sinners are of two kinds: there are some that have received Law, there are others that have not received: all the heathen have not received Law, all Jews and Christians have received Law. Therefore the general term is sinner; either a transgressor of the Law, if he has received Law; or only unjust without Law, if he has not received the Law. Of both kinds speaks the Apostle, and says, “They that without Law have sinned, without Law shall perish, and they that in the Law have sinned, by the Law shall be judged.” But thou that amid both kinds dost groan, say to God that which you hear in the Psalm, “My God, deliver me from the hand of the sinner.” Of what sinner? “From the hand of him that transgresses the Law, and of the unjust man.” He that transgresses the Law is indeed also unjust; for not unjust he is not, that transgresses the Law: but every one that transgresses the Law is unjust, not every unjust man does transgress the Law. For, “Where there is not a Law,” says the Apostle, “neither is there transgression.” They then that have not received Law, may be called unjust, transgressors they cannot be called. Both are judged after their deservings. But I that from captivity will to be delivered through Your grace, cry to You, “Deliver me from the hand of the sinner.” What is, from the hand of him? From the power of him, that while he is raging, he lead me not unto consenting with him; that while he lies in wait, he persuade not to iniquity. “From the hand of the sinner and of the unjust man.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)