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.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)