On Psalm 1.
“Blessed is the man that hath not gone away in the counsel of the ungodly” (ver. 1). This is to be understood of our Lord Jesus Christ, the Lord-Man.
“Blessed is the man that hath not gone away in the counsel of the ungodly,” as “the man of earth did” (1 Cor. 15:47), who consented to his wife deceived by the serpent to the transgressing the commandment of God.
“Nor stood in the way of sinners.” For He came indeed in the way of sinners, by being born as sinners are; but He “stood” not therein, inasmuch as the enticements of the world held Him not.
“And hath not sat in the seat of pestilence.” He willed not an earthly kingdom, with pride, which is well taken for “the seat of pestilence.”
For there is hardly anyone who is free from the love of rule, and craves not human glory. For a “pestilence” is disease widely spread, and involving all or nearly all.
[…] “And he shall be like a tree planted hard by the running streams of waters” (ver. 3). This refers either to True “Wisdom” (Prov. 8), which vouchsafed to assume man’s nature for our salvation; that as man He might be “the tree planted hard by the running streams of waters.”
This can be understood in the sense which we find in another Psalm: “the river of God is full of water” (Ps. 64:9). Or it can be taken as referring to the Holy Spirit, of whom it is said, “He shall baptize you in the Holy Spirit” (Matt. 3:11), and again, “If any man thirst, let him come unto Me, and drink” (John 7:37).
[…] Alternatively, “by the running streams of waters” may mean “by the sins of the people”, because, firstly, the waters are called “peoples” in the Apocalypse (Rev. 17:15); and again, “running stream” may reasonably be understood as meaning “waterfall,” which can refer to sin.
That “tree” (that is, our Lord), drawing them from the running streams of water (that is, from the sinful peoples by the way) into the roots of His discipline, will “bring forth fruit,” that is, will establish Churches, “in His season,” that is, after He has been glorified by His Resurrection and Ascension into heaven.
For then, by the sending of the Holy Spirit to the Apostles, and by the confirming of their faith in Him, and their mission to the world, He made the Churches to “bring forth fruit.” “His leaf also shall not fall,” that is, His Word shall not be in vain.
Augustine of Hippo (354-430): Exposition of the Book of Psalms, Psalm 1, 1,3 (slightly adapted).