8 “Take heed now, you that are unwise among the people: O you fools, some time understand!”. He teaches His people whose feet might slip: any one among them sees the prosperity of the wicked, himself living well among the Saints of God, that is, among the number of the sons of the Church: he sees that the wicked flourish, and work iniquity, he envies, and is led to follow them in their actions; because he sees that apparently it profits him nothing that he lives well in humility, hoping for his reward here. For if he hopes for it in future, he loses it not; because the time is not yet come for him to receive it. You are working in a vineyard: execute your task, and you shall receive your pay. You would not exact it from your employer, before your work was finished, and yet do you exact it from God before thou dost work? This patience is part of your work, and your pay depends upon your work: thou who dost not choose to be patient, choosest to work less upon the vineyard: since this act of patience belongs to your labouring itself, which is to gain your pay. But if you are treacherous, take care, lest you should not only not receive your pay, but also suffer punishment, because you have chosen to be a treacherous labourer. When such a labourer begins to do ill, he watches his employer's eyes, who hired him for his vineyard, that he may loiter when his eye is turned away; but the moment his eyes are turned towards him, he works diligently. But God, who hired you, averts not His eyes: you can not work treacherously: the eyes of your Master are ever upon you: seek an opportunity to deceive Him, and loiter if you can. If then any of you had any such ideas, when you saw the wicked flourishing, and if such thoughts caused your feet to slip in the path of God; to you this Psalm speaks: but if perchance none of you be such, through you it does address others, in these words, “Take heed now;” since they had said, “The Lord shall not see: neither shall the God of Jacob regard it.” “Take heed,” it says, “now, you that are unwise among the people: and you fools, some time understand!”
9. “He that planted the ear, shall He not hear? Or He that made the eye, does He not consider?” “or He that instructs the nations, shall He not reprove?”. This is what God is at present doing: He is instructing the nations: for this reason he sent His word to man throughout the world: He sent it by Angels, by Patriarchs, by Prophets, by servants, through so many heralds going before the Judge. He sent also His own Word Himself, He sent His own Son in Person: He sent the servants of His Son, and in these very servants His own Son. Throughout the world is everywhere preached the word of God. Where is it not said unto men, Abandon your former wickedness, and turn yourselves to right paths? He spares, that you may correct yourselves: He punished not yesterday, in order that today ye may live well. He teaches the heathen, shall He not therefore reprove? will He not hear those whom He teaches? will He not judge those to whom He has beforehand sent and sown lessons of warning? If you were in a school, would you receive a task, and not repeat it? When therefore you receive it from your master, you are being taught: the Master gives your task into your hands, and shall He not exact it from you when you come to repeat it? Or when you have begun to repeat it, shall you not be in fear of stripes? At present then we are receiving our work: afterwards we are placed before the Master, that we may give up to Him all our past tasks, that is, that we may give an account of all those things which are now being bestowed upon us. Hear the Apostle's words: “We must all appear before the judgment-seat of Christ,” etc. “It is He that teaches man knowledge.” Does He not know, who makes you to know?
10. “The Lord knows the thoughts of man, that they are but vain”. For although you know not the thoughts of God, that they are righteous; “He knows the thoughts of man, that they are but vain.” Even men have known the thoughts of God: but those to whom He has become a friend, it is to them He shows His counsel. Do not, brethren, despise yourselves: if you approach the Lord with faith, you hear the thoughts of God; these you are now learning, this is told you, and for this reason you are taught, why God spares the wicked in this life, that you may not murmur against God, who teaches man knowledge. “The Lord knows the thoughts of man, that they are but vain.” Abandon therefore the thoughts of man, which are vain: that you may take hold on the thoughts of God, which are wise. But who is he who takes hold on the thoughts of God? He who is placed in the firmament of heaven. We have already chanted that Psalm, and have expounded this expression therein.
11. “Blessed is the man whom You chasten, O Lord: and teachest him from Your law”. Behold, you have the counsel of God, wherefore He spares the wicked: the pit is being dug for the sinner. You wish to bury him at once: the pit is as yet being dug for him: do not be in haste to bury him. What mean the words, “until the pit be dug up for the sinner”? Or whom does He mean by sinner? One man? No. Whom then? The whole race of such that are sinners? No; them that are proud; for he had said before, “Reward the proud after their deserving.” For that publican, who would not so much as lift up his eyes to heaven, but “smote upon his breast, saying, God be merciful to me a sinner,” was a sinner; but since he was not proud, and since God will render a recompense to the proud; the pit is being dug not for him, but for them that are such, until He render a recompense to the proud. In the words then, “until the pit be dug up for the ungodly,” understand the proud. Who is the proud? He who does not by confession of his sins do penance, that he may be healed through his humility. Who is the proud? He who chooses to arrogate to himself those few good things which he seems to possess, and who does detract from the mercy of God. Who is the proud? He who although he does ascribe unto God his good works, yet insults those who do not those good works, and raises himself above them....This then is the Christian doctrine: no man does anything well except by His grace. A man's bad acts are his own: his good he does of God's bounty. When he has begun to do well, let not him ascribe it unto himself: when he has not attributed it to himself, let him give thanks to Him from whom he has received it. But when he does well, let him not insult him who does not as he does nor exalt himself above him: for the grace of God is not stayed at him, so that it cannot reach another.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)