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!”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)