1 I have not thought that the CVIIIth Psalm required an exposition; since I have already expounded it in the LVIIth Psalm, and in the LXth, of the last divisions of which this Psalm consists. For the last part of the LVIIth is the first of this, as far as the verse, “Your glory is above all the earth.” Henceforth to the end, is the last part of the LXth: as the last part of the CXXXVth is the same as that of the CXVth, from the verse, “The images of the heathen are but gold and silver:” as the XIV and LIIId, with a few alterations in the middle, have everything the same from the beginning to the end. Whatever slight differences therefore occur in this CVIIIth Psalm, compared with those two, of parts of which it is composed, are easy to understand; just as we find in the LVIIth, “I will sing and give praise; awake, O my glory:” here,“I will sing and give praise, with my glory.” Awake, is said there, that he may sing and give praise therewith. Also, there, “Your mercy is great” (or, as some translate, “is lifted up”) “unto the heavens;” but here, “Your mercy is great above the heavens.” For it is great unto the heavens, that it may be great in the heavens; and this is what he wished to express by “above the heavens.” Also in the LXth, “I will rejoice, I will divide Shechem:” here “I will be exalted, and will divide Shechem.” Where is shown what is signified in the division of Shechem, which it was prophesied should happen after the Lord's exaltation, and that this joy does refer to that exaltation; so that He rejoices, because He is exalted. Whence he elsewhere says, “You have turned my heaviness into joy; You have put off my sackcloth and girded me with gladness.” Also there “Ephraim, the strength of my head:” but here, “Ephraim the taking up of my head.” But strength comes from taking up, that is, He makes men strong by taking up, causing fruit in us; for the interpretation of Ephraim is, bearing fruit. But “taking up” may be understood of us, when we take up Christ; or of Christ, when He, who is Head of the Church, takes us up. And the words, “them that trouble us,” in the former Psalm, are the same with “our enemies,” in this.
2. We are taught by this Psalm, that those titles which seem to refer to history are most rightly understood prophetically, according to the object of the composition of the Psalms....And yet this Psalm is composed of the latter portions of two, whose titles are different. Where it is signified that each concur in a common object, not in the surface of the history, but in the depth of prophecy, the objects of both being united in this one, the title of which is, “A Song or Psalm of David:” resembling neither of the former titles, otherwise than in the word David. Since, “in many places, and in diverse manners,” as the Epistle to the Hebrews says, “God spoke in former times to the fathers through the Prophets;” yet He spoke of Him whom He sent afterwards, that the words of the Prophets might be fulfilled: for “all the promises of God in Him are yea.”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)