5 The Holy Spirit made choice of this magnificent and noble introduction to the Psalter, in order to stir up weak man to a pure zeal for piety by the hope of happiness, to teach him the mystery of the Incarnate God, to promise him participation in heavenly glory, to declare the penalty of the Judgment, to proclaim the two-fold resurrection, to show forth the counsel of God as seen in His award. It is indeed after a faultless and mature design that He has laid the foundation of this great prophecy; His will being that the hope connected with the happy man might allure weak humanity to zeal for the Faith; that the analogy of the happiness of the tree might be the pledge of a happy hope, that the declaration of His wrath against the ungodly might set the bounds of fear to the excesses of ungodliness, that difference in rank in the assemblies of the saints might mark difference in merit, that the standard appointed for judging the ways of the righteous might show forth the majesty of God.
But let us now deal with the subject matter and the words which express it.
Source: Homilies on the Psalms (New Advent)