7 Because of this there follows in the Psalm, “You have loved all words of sinking under”. Rescue therefore yourself, if you can, from sinking under. From shipwreck you are fleeing, and dost embrace lead! If you will not sink, catch at a plank, be borne on wood, let the Cross carry you through. But now because you are a Doeg the Edomite, a “motion,” and “of earth,” you do what? “You have loved all words of sinking-under, a tongue deceitful.” This has preceded, words of sinking-under have followed a tongue deceitful. What is a tongue deceitful? A minister of guile is a tongue deceitful, of men bearing one thing in heart, another thing from mouth bringing forth. But in these is overthrowing, in these sinking under.
8. “Wherefore God shall destroy you at the end”: though now you seem to flourish like grass in the field before the heat of the sun. For, “All flesh is grass, and the brightness of man as the bloom of grass: the grass has withered, and the bloom has fallen down: but the word of the Lord abides for everlasting.” Behold that to which you may bind yourself, to what “abides for everlasting.” For if to grass, and to the bloom of grass, you shall have bound yourself, since the grass shall wither, and the bloom shall fall down, “God shall destroy you at the end:” and if not now, certainly at the end He shall destroy, when that winnowing shall have come, and the heap of chaff from the solid grain shall have been separated. Is not the solid grain for the barns, and the chaff for the fire? Shall not the whole of that Doeg stand at the left hand, when the Lord is to say, “Go ye into fire everlasting, which has been prepared for the devil and his angels”? Therefore “God shall destroy at the end: shall pluck you out, and shall remove you from your dwelling.” Now then this Doeg the Edomite is in a dwelling: “But a servant abides not in the house for ever.” Even he works something of good, even if not with his doings, at least with the words of God, so that in the Church, when he “seeks his own,” he would say, at least, those things which are of Christ.
“But He shall remove you from your dwelling.” “Verily, verily, I say unto you, they have received their reward.” “And your root from the land of the living.” Therefore in the land of the living we ought to have root. Be our root there. Out of sight is the root: fruits may be seen, root cannot be seen. Our root is our love, our fruits are our works: it is needful that your works proceed from love, then is your root in the land of the living. Then shall be rooted up that Doeg, nor any wise shall he be able there to abide, because neither more deeply there has he fixed a root: but it shall be with him in like manner as it is with those seeds on the rock, which even if a root they throw out, yet, because moisture they have not, with the risen sun immediately do wither. But, on the other hand, they that fix a root more deeply, hear from the Apostle what? “I bow my knees for you to the Father of our Lord Jesus Christ, that you may be in love rooted and grounded.” And because there now is root, “That ye may be able,” he says, “to comprehend what is the height, and breadth, and length, and depth: to know also the supereminent knowledge of the love of Christ, that you may be filled unto all the fullness of God.” Of such fruits so great a root is worthy, being so single, so budding, for buddings so deeply grounded. But truly this man's root shall be rooted up from the land of the living.
9. “And the just shall see, and shall fear; and over him they shall laugh”. Shall fear when? Shall laugh when? Let us therefore understand, and make a distinction between those two times of fearing and laughing, which have their several uses. For so long as we are in this world, not yet must we laugh, lest hereafter we mourn. We have read what is reserved at the end for this Doeg, we have read and because we understand and believe, we see but fear. This, therefore, has been said, “The just shall see, and shall fear.” So long as we see what will result at the end to evil men, wherefore do we fear? Because the Apostle has said, “In fear and trembling work out your own salvation:” because it has been said in a Psalm, “Serve the Lord in fear, and exult unto Him with trembling.” Wherefore “with fear”? “Wherefore let him that thinks himself to stand, see that he fall not.” Wherefore “with trembling”? Because he says in another place: “Brethren, if a man shall have been overtaken in any delinquency, you that are spiritual instruct such sort in the spirit of gentleness; heeding yourself, lest you also be tempted.” Therefore, the just that are now, that live of faith, so see this Doeg, what to him is to result, that nevertheless they fear also for themselves: for what they are today, they know; what tomorrow they are to be, they know not. Now, therefore, “The just shall see, and they shall fear.” But when shall they laugh? When iniquity shall have passed over; when it shall have flown over; as now to a great degree has flown over the time uncertain; when shall have been put to flight the darkness of this world, wherein now we walk not but by the lamp of the Scriptures, and therefore fear as though in night. For we walk by prophecy; whereof says the Apostle Peter, “We have a more sure prophetic word, to which giving heed ye do well, as to a lamp shining in a dark place, until the day shine, and the day-star arise in your hearts.” So long then as by a lamp we walk, it is needful that with fear we should live. But when shall have come our day, that is, the manifestation of Christ, whereof the same Apostle says, “When Christ shall have appeared, your life, then ye also shall appear with Himself in glory,” then the just shall laugh at that Doeg....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)