2 The Title of the Psalm is: “Unto the end, in behalf of those that shall be changed, to David himself.” Now of the change for the better hear thou; for change either is for the worse or for the better....That we have been changed then for the worse, to ourselves let us ascribe: that for the better we are changed, let us praise God. “For those,” then, “that shall be changed,” this Psalm is. But whence has this change been made but by the Passion of Christ? The very word Pascha in Latin is interpreted passage. For Pascha is not a Greek word but a Hebrew. It sounds indeed in the Greek language like Passion, because π·σχειν signifies to suffer: but if the Hebrew expression be examined, it points to something else. Pascha does intimate passage. Of which even John the Evangelist has admonished us, who (just before the Passion when the Lord was coming to the supper wherein He set forth the Sacrament of His Body and Blood) thus speaks: “But when there had come the hour, wherein Jesus was to pass from this world to the Father.” He has expressed then the “passage” of the Pascha. But unless He passed Himself hence to the Father, who came for our sake, how should we have been able to pass hence, who have not come down for the sake of taking up anything, but have fallen? But He Himself fell not; He but came down, in order that He might raise up him that had fallen. The passage therefore both of Him and of us is hence to the Father, from this world to the kingdom of Heaven, from life mortal to life everlasting, from life earthly to life heavenly, from life corruptible to life incorruptible, from intimacy with tribulations to perpetual security. Accordingly, “In behalf of them that shall be changed,” the Psalm's title is. The cause therefore of our change, that is, the very Passion of the Lord and our own voice in tribulations in the text of the Psalm let us observe, let us join in knowing, join in groaning, and in hearing, in joint-knowing, joint-groaning, let us be changed, in order that there may be fulfilled in us the Title of the Psalm, “In behalf of them that shall be changed.”
3. “Save me, O God, for the waters have entered in even unto my soul”. That grain is despised now, that seems to give forth humble words. In the garden it is buried, though the world will admire the greatness of the herb, of which herb the seed was despised by the Jews. For in very deed observe ye the seed of the mustard, minute, dull coloured, altogether despicable, in order that therein may be fulfilled that which has been said, We have seen Him, and He had neither form nor comeliness. But He says, that waters have come in even unto His soul; because those multitudes, which under the name of waters He has pointed out, were able so far to prevail as to kill Christ....Whence then does He so cry out, as though He were suffering something against His will, except because the Head does prefigure the Members? For He suffered because He willed: but the Martyrs even though they willed not; for to Peter thus He foretold his passion: “When you shall be old,” He says, “another shall gird you, and lead you whither you will not.” For though we desire to cleave to Christ, yet we are unwilling to die: and therefore willingly or rather patiently we suffer, because no other passage is given us, through which we may cleave to Christ. For if we could in any other way arrive at Christ, that is, at life everlasting, who would be willing to die? For while explaining our nature, that is, a sort of association of soul and body, and in these two parts a kind of intimacy of gluing and fastening together, the Apostle says, that “we have a House not made with hands, everlasting in the Heavens:” that is, immortality prepared for us, wherewith we are to be clothed at the end, when we shall have risen from the dead; and he says, “Wherein we are not willing to be stripped, but to be clothed upon, that the mortal may be swallowed up of life.” If it might so be, we should so will, he says, to become immortal, as that now that same immortality might come, and now as we are it should change us, in order that this our mortal body by life should be swallowed up, and the body should not be laid aside through death, so as at the end again to have to be recovered. Although then from evil to good things we pass, nevertheless the very passage is somewhat bitter, and has the gall which the Jews gave to the Lord in the Passion, has something sharp to be endured, whereby they are shown that gave Him vinegar to drink....For here both sweet are temporal pleasures, and bitter are temporal tribulations: but who would not drink the cup of tribulation temporal, fearing the fire of hell; and who would not contemn the sweetness of the world, longing for the sweetness of life eternal? From hence that we may be delivered let us cry: lest perchance amidst oppressions we consent to iniquity, and truly irreparably we be swallowed up.
4. “Fixed I am in the clay of the deep, and there is no substance”. What called the clay? Is it those very persons that have persecuted? For out of clay man has been made. But these men by falling from righteousness have become the clay of the deep, and whosoever shall not have consented to them persecuting and desiring to draw him to iniquity, out of his clay does make gold. For the clay of the same shall merit to be converted into a heavenly form, and to be made associate of those of whom says the Title of the Psalm, “in behalf of them that shall be changed.” But at the time when these were the clay of the deep, I stuck in them: that is, they held Me, prevailed against Me, killed Me. “Fixed” then “I am in the clay of the deep, and there is no substance.” What is this, “there is no substance”? Can it be that clay itself is not a substance? What is then, “fixed I am”? Can it be that Christ has thus stuck? Or has He stuck, and was not, as has been said in the book of Job, “the earth delivered into the hands of the ungodly man”? Was He fixed in body, because it could be held, and suffered even crucifixion? For unless with nails He had been fixed, crucified He had not been. Whence then “there is no substance”? Is that clay not a substance? But we shall understand, if it be possible, what is, “and there is no substance,” if first we shall have understood what is a substance. For there is substance spoken of even of riches, as we say, he has substance, and he has lost substance....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)