Thomas Aquinas, on Ps 36:18

“The Lord knoweth the days of undefiled; and their inheritance shall be for ever.”

“Next, when he says, "The Lord knows," he makes his proposition clear. And first he speaks of the stability of the just. Second, of the crushing of the unjust, at "For sinners." Now the just are strengthened by God in two ways: first, their goods are made stable; second, they are cured of evils, at "They will not be confounded." But there is a twofold good, namely the good of the way and the good of the end; and in both the just are made stable. Regarding the first he says, "The Lord knows." Regarding the second he says, "And their inheritance." He says therefore, "The Lord knows": Heb. 4: "All things are naked and open to his eyes." But especially what is akin and familiar to him: 2 Tim. 2: "The Lord knows those who are his," and those whom he approves. And thus it is said here, "The Lord knows," that is, approves, "the days of the blameless." But Job 15 says: "Who is man that he should be blameless, and that he should be just?" etc. The response is that it is true that of himself no one is blameless, if by "blemish" is understood mortal sin; but through grace, they are. If, however, by "blemish" is understood original sin, then no one is blameless. These "days" can be understood in three ways. First, as the days of the present life; and although they are common to good and evil, yet the good make good use of them: Gen. 25: "He died in a good old age, advanced in years and full of days." But the wicked make bad use of them: Ps. 54: "Men of blood and deceit will not live out half their days." And because the days of the just are full and therefore approved, while those of the wicked are cut short and therefore not approved. Likewise, these days are evil and few, yet God knows them. Others are the works of virtue: Rom. 13: "Let us walk honestly as in the day"; and these are approved by God. Others are the days of eternity and justice; and these are the days of the just, known to God alone: Is. 64: "Eye has not seen, apart from you, what you have prepared for those who wait for you." The good of the end he shows when he says, "And their inheritance will be forever." That is called an inheritance in which there is the establishment and end of someone. The just have their end in an eternal thing: Ps. 15: "The Lord is the portion of my inheritance": Lam. 3: "My portion." And therefore their inheritance cannot fail. But the wicked place their end in worldly things: Wis. 2: "This is our portion," etc., and therefore it does not endure. And of these it is said, 1 Pet. 1: "Unto an inheritance preserved in heaven."”

Imported from an open dataset — not yet checked against the printed edition.