Ps 36:12

“(Vers. 12, 13.) It follows: The sinner will observe the just and will gnash his teeth over him. But the Lord will deride him; for He sees that his day is coming. It is customary for one who is angry and seething to gnash his teeth: but the heart of the wicked also has its teeth, which are not accustomed to merely making noise, but to tearing. The sinner's traps, schemes, and wickedness are his teeth. Therefore, the sinner lies in wait for the just, because he envies him; for the life of the just rebukes the sinner, whom it silently condemns with greater authority than if it were to speak with a loud voice. But the righteous should not fear the clamor of the sinner, for wickedness cannot be everlasting. Temporary are the snares, but eternal are the fortresses of virtue. Death dissolves all power and deceit of the sinner.”

“The way in which consolation is offered to us is amazing, for who should delight himself in the pompous display of one whose audacity he knows will soon perish? If we would rather not be disturbed by jealousy, then let us follow what the Lord does. Let us laugh at him whose downfall we foresee; let us regard the unhappy things which we possess as things that will perish in time. Let us believe these things with all tenacity, since they have been promised to us in truth. It will happen in the way described, that the sinner will depart under derision, although he once boasted in a happiness that was not to last.”

“"He will watch." Above he set forth one reason why the men of God ought not be indignant over the sinner, namely because his prosperity is not stable; here he sets forth another, namely that sinners cannot harm the just. And regarding this he does two things. First, he proposes the malice that they devise in their heart against the just. Second, their outward effort, at "They have drawn the sword." Regarding the first, he does two things, inasmuch as their interior malice is twofold. First, he shows that the wicked lie in wait for the good. Second, that they are disturbed against them, at "And he will gnash." He says therefore, "The sinner will watch the just man," to see if perhaps he might find something against him: Lk. 14: "And they were watching him." So sinners watch the good, lying in wait for them and perverting their holy endeavors. Those who turn the goods of the Church into evils lie in wait, etc. But against this it is said, Prov. 24: "Do not lie in wait, and do not seek wickedness in the house of the just," etc. Second, they are disturbed against the just man. For they say, Wis. 2: "His life is unlike that of others," etc. Hence, "He will gnash his teeth at him," that is, they will be moved to anger. He speaks after the manner of boars: Ps. 111: "The sinner shall see, and shall be angry": Acts 7: "They gnashed their teeth at him."”