3 For it goes on, “For the things that shall be changed, to the sons of Korah for understanding; a song for the beloved.” For that “beloved” One was seen by His persecutors, but yet not for “understanding.” For “had they known Him, they would never have crucified the Lord of Glory.” In order to this “understanding,” other eyes were required by Him when He said, “He that sees Me, sees My Father also.” Let the Psalm then now sound of Him, let us rejoice in the marriage-feast, and we shall be with those of whom the marriage is made, who are invited to the marriage; and the very persons invited are the Bride herself. For the Church is “the Bride,” Christ the Bridegroom. There are commonly spoken by balladists certain verses to Bridegrooms and Brides, called Epithalamia. Whatever is sung there, is sung in honour of the Bride and Bridegroom. Is there then no Bridechamber in that marriage-feast to which we are invited? Whence then does another Psalm say, “He has set up His tabernacle in the Sun; and He is even as a bridegroom coming out of his chamber.” The nuptial union is that of “the Word,” and the flesh. The Bridechamber of this union, the Virgin's womb. For the flesh itself was united to the Word: whence also it is said, “Henceforth they are not two, but one flesh.” The Church was assumed unto Him out of the human race: so that the Flesh itself, being united to the Word, might be the Head of the Church: and the rest who believe, members of that Head....
4. “Mine heart has uttered a good word”. Who is the speaker? The Father, or the Prophet? For some understand it to be the Person of the Father, which says, “Mine heart has uttered a good word,” intimating to us a certain unspeakable generation. Lest you should haply think something to have been taken unto Him, out of which God should beget the Son (just as man takes something to himself out of which he begets children, that is to say, an union of marriage, without which man cannot beget offspring), lest then you should think that God stood in need of any nuptial union, to beget “the Son,” he says, “Mine heart has uttered a good word.” This very day your heart, O man, begets a counsel, and requires no wife: by the counsel, so born of your heart, you build something or other, and before that building subsists, the design subsists; and that which you are about to produce, exists already in that by which you are going to produce it; and you praise the fabric that as yet is not existing, not yet in the visible form of a building, but on the projecting of a design: nor does any one else praise your design, unless either you show it to him, or he sees what you have done. If then by the Word “all things were made,” and the Word is of God, consider the fabric reared by the Word, and learn from that building to admire His counsels! What manner of Word is that by which heaven and earth were made; and all the splendour of the heavens; all the fertility of the earth; the expanse of the sea; the wide diffusion of air; the brightness of the constellations; the light of sun and moon? These are visible things: rise above these also; think of the Angels, “Principalities, Thrones, Dominions, and Powers.” All were made by Him. How then were these good things made? Because there was “uttered forth 'a good Word,'” by which they were to be made....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)