8 There have not been wanting those who preferred understanding all the preceding passage also of the Prophet's own person; and would have even this verse, “Mine heart has uttered forth a good word,” understood as spoken by the Prophet, supposed to be uttering a hymn. For whoever utters a hymn to God, his heart is, as it were, “uttering forth a good word,” just as his heart who blasphemes God, is uttering forth an evil word. So that even by what follows, “I speak of the things which I have made unto the King,” he meant to express that man's chief work was but to praise God. To Him it belongs to satisfy you, by His beauty; to you to praise Him with thanksgiving....
9. “My tongue is the pen of a writer writing quickly.” There have been persons who have understood the Prophet to have been describing in this manner what he was writing; and therefore to have compared his tongue to “the pen of a writer writing quickly:” but that he chose to express himself in the words “writing quickly,” to signify, that he was writing of things which were to come “quickly;” that “writing quickly” should be understood to be equivalent to “writing things that are quick;” i.e. writing things that would not long tarry. For God did not tarry long to manifest Christ. How quickly is that perceived to have rolled by, which is acknowledged to be already past! Call to mind the generations before you; you will find that the making of Adam is but a thing of yesterday. So do we read that all things have gone on from the very beginning: they were therefore done “quickly.” The day of Judgment also will be here “quickly.” Do thou anticipate its “quick” coming. It is to come “quickly;” do thou become converted yet more “quickly.” The Judge's face will appear: but observe thou what the Prophet says, “Let us come before” (let us “prevent”) “His face with confession.”
10. “Gird Your sword upon Your thigh, O most Mighty”. What is meant by “Your sword,” but “Your word”? It was by that sword He scattered His enemies; by that sword he divided the son from the father, “the daughter from the mother, the daughter-in-law from the mother-in-law.” We read these words in the Gospel, “I came not to send peace, but a sword.” And, “In one house shall five be divided against each other; three against two, and two against three;” i.e. “the father against the son, the daughter against the mother, the daughter-in-law against the mother-in-law.” By what “sword,” but that which Christ brought, was this division wrought? And indeed, my brethren, we see this exemplified daily. Some young man is minded to give himself up to God's service; his father is opposed to it; they are “divided against each other:” the one promises an earthly inheritance, the other loves an heavenly; the one promises one thing, the other prefers another. The father should not think himself wronged: God alone is preferred to him. And yet he is at strife with the son, who would fain give himself to God's service. But the spiritual sword is mightier to separate them, than the ties of carnal nature to bind them together. This happens also in the case of a mother against her daughter; still more also in that of a daughter-in-law against a mother-in-law. For sometimes in one house mother-in-law and daughter-in law are found orthodox and heretical respectively. And where that sword is forcibly felt, we do not dread the repetition of Baptism. Could daughter be divided against mother; and could not daughter-in-law be divided against mother-in-law?...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)