18 “Will You have anything to do with the stool of iniquity, who makest sorrow in learning?”. He has said this, No wicked man sits with You, nor shall Thou have anything to do with the stool of iniquity. And he gives an account whereof he understands this, “For You make sorrow in learning.” For from this, because You have not spared us, do I understand that You have nothing to do with the stool of iniquity. You have this in the Epistle of the Apostle Peter, and for this reason he has adduced a testimony from the Scripture: “for the time has come,” he says, “that judgment must begin at the house of God;” that is, the time has come for the judgment of those who belong to the house of God.
If sons are scourged, what must the most wicked slaves expect? For which reason he added: “And if it first begin at us, what shall the end be of them that obey not the Gospel of God?” To which he added this testimony: “For if the righteous scarcely be saved, where shall the ungodly and sinner appear?” How then shall the wicked be with You, if Thou dost not even spare Your faithful, in order that You may exercise and teach them? But as He spares them not, for this reason, that He may teach them: he says, “For You make sorrow in learning.”
“Makest,” that is, formest: from whence comes the word figulus (from fingo), and a potter's vessel is called fictile: not in the meaning of fiction, a falsehood, but of forming so as to give anything being and some sort of form; as before he said, “He that fabricated (finxit) the eye, shall He not see?” Is that, “fabricated the eye” a falsehood? Nay, it is understood He fashioned the eye, made the eye. And is He not a potter when He makes men frail, weak, earthly?
Hear the Apostle: “We have this treasure in earthen vessels.”...Behold our Lord Himself, how He shows Himself a potter. Because He had made man of clay, He anointed him with clay, for whom He had not made eyes in the womb. And so when he says, “Have You anything to do,” etc., he says, out of grief makest learning for us, so that grief itself becomes our instruction. How is sorrow our learning? When He scourges you who died for you, and who does not promise bliss in this life, and who cannot deceive, and when He gives not here what you seek.
What will He give? When will He give? How much will He give, who gives not here, who here teaches, who makes sorrow in learning? Your labour is here, and rest is promised you. You take thought that you have toil here: but take thought what sort of rest He promises. Can you conceive it? If you could, you would see that your toil here is nothing toward an equivalent....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)