2 What this Psalm contains, I believe that you perceived when it was being chanted; for therein the Church of Christ, set in the midst of the wicked, complains and groans, and pours out prayer to God. For her voice is in every such prophecy the voice of one in need and want, not yet satisfied, “hungering and thirsting after righteousness,” for whom a certain fullness in the end has been promised, and is reserved....
3. “To the end, a Psalm to David himself.” No other end may thou look to, than is laid down for you by the Apostle himself. For “Christ is the end.”...He was of the seed of David, not after His Godhead, whereby He is the Creator of David, but after the flesh; therefore He deigned to be called David in prophecy: look to this “end,” for the Psalm is chanted “to David Himself;” hear the voice of His Body; be in His Body. Let the voice which you have heard be yours, and pray, and say what follows.
4. “Deliver me, O Lord, from the wicked man”. Not from one only, but from the class; not from the vessels only, but from their prince himself, that is, the devil. Why “from man,” if he means from the devil? Because he too is called a man in a figure....Now then being made light, not in ourselves, but in the Lord, let us pray not only against darkness, that is, against sinners, whom still the devil possesses, but also against their prince, the devil himself, who works in the children of disobedience. “Deliver me from the unrighteous man.” The same as “from the wicked man.” For he called him wicked because unrighteous, lest perchance you should think that any unrighteous man could be a good man. For many unrighteous men seem to be harmless; they are not fierce, are not savage, do not persecute nor oppress; yet are they unrighteous, because, following some other habit, they are luxurious, drunkards, given to pleasure....Wicked then is every unrighteous man, who must needs be harmful, whether he be gentle or fierce. Whoever falls in his way, whoever is taken by his snares, will find how harmful is that which he thought harmless. For, brethren, even thorns prick not with their roots. Pull up thorns from the ground, handle their roots, and see whether you feel pain. Yet that in the upgrowth which causes you pain, proceeded from that root. Let not then men please you who seem gentle and kind, yet are lovers of carnal pleasure, followers of polluted lusts, let them not please you. Though as yet they seem gentle, they are roots of thorns....And so, my brethren, body of Christ, members of Christ groaning among such wicked men, whomsoever ye find hurrying headlong into evil lusts and deadly pleasures, at once chide, at once punish, at once burn. Let the root be burnt, and there remains not whence the thorn may grow up. If you cannot, be sure that you will have them as enemies. They may be silent, they may hide their enmity, but they cannot love you. But since they cannot love you, and since they who hate you must needs seek your harm, let not your tongue and heart be slow to say to God, “Deliver me, O Lord, from the unrighteous man.”
5. “Who have imagined unrighteousnesses in their heart”....From them free me, from them let Your hand be most powerful to deliver me. For easy is it to avoid open enmities, easy is it to turn aside from an enemy declared and manifest, while iniquity is in his lips as well as his heart; he is a troublesome enemy, he is secret, he is with difficulty avoided, who bears good things in his lips, while in his heart he conceals evil things. “All the day long did they make war.” What is, “war”? They made for me what I was to fight against all the day. For from thence, from such hearts as these, arises all that the Christian fights against. Be it sedition, be it schism, be it heresy, be it turbulent opposition, it springs not save from these imaginings which were concealed, and while they spoke good words with their lips, “all the day long did they make war.” You hear words of peace, yet making war departs not from their thoughts. For the words, “all the day long,” signify without intermission, throughout the whole time. “They have sharpened their tongues like serpents”. If still you seek to make out the man, behold a comparison. In the serpent above all beasts is there cunning and craft to hurt; for therefore does it creep. It has not even feet, so that its footsteps when it comes may be heard. In its progress it draws itself, as it were, gently along, yet not straightly. Thus then do they creep and crawl to hurt, having poison hidden even under a gentle touch. And so it follows, “the poison of asps is under their lips.” Behold, it is “under” their lips, that we may perceive one thing under their lips, another in their lips....
6. “Preserve me, O Lord, from the hand of the sinner, from unrighteous men deliver me”. Here they wear their real colours, they are known; here we have no need to understand, but to act: we have need to pray, not to ask who they are. But how you should pray against such men, he explains in what follows. For many pray unskilfully against wicked men. “Who have imagined,” says he, “to trip up my steps.” Thus far it may be understood carnally. Every one has enemies, who seek to cheat him in trade, to rob him of money, where they are engaged together in business; every one has some neighbour his enemy, who devises how to bring mischief upon his family, to injure in some way his property and surely he devises this by deceit, by fraud, by devilish devices he endeavours to accomplish this: no one can doubt it. Yet not for these reasons are they to be guarded against, but lest they lay in wait for you and draw you to themselves, that is, separate you from the Body of Christ, and make you of their body. For as Christ is the Head of the good, so is the devil their head. What is, “to trip up my steps”? Not as though you should be deceived in the business you have with him, or he cheat you in a case which you have with him in the law courts. He has “tripped up your steps,” if he have hindered you in the way of God; so that what you directed aright may stumble, or fall from the way, or fall in the way, or draw back from the way, or stop on the way, or go back to the place from whence it had come. Whatsoever has done this to you, has tripped you up, has deceived you. Against such snares as these pray thou, lest you lose your heavenly inheritance, lest you lose Christ your Joint-heir, for you are destined to live for ever with Him, who has made you an heir. For you are made an heir, not by one whom you are to succeed after his death, but One together with whom you are to live for ever.
7. “The proud have hidden a trap for me”. He has briefly described the whole body of the devil, when he says, “the proud.” Hence is it that for the most part they call themselves righteous when they are unrighteous. Hence is it that nothing is so grievous to them as to confess their sins. They are men who, being falsely righteous, must needs envy the truly righteous. For none envies another in that which he wishes not either to be or to seem....Hence come all allurings and trippings up of others. This the devil first wished, when falling himself he envied man who stood....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)