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)