36 “O remember what my substance is”. That David, who was placed among the Jews in the flesh, in Christ in hope, speaks “Remember what is my substance.” For not because the Jews fell away, did my substance fail: for from that people came the Virgin Mary, and from her the flesh of Christ; that Flesh sins not, but purifies sins; there, says David, is my substance. “O remember what my substance is.” For the root has not entirely perished; the seed shall come to whom the promise was made, ordained by Angels in the hand of a Mediator. “For You have not made all the sons of men for nought”.
Lo! All the sons of men have gone into vanity: yet You have not made them for nought. If then all went into vanity, whom You have not made for nought; have You not reserved some instrument to purify them from vanity? This which You have reserved to Yourself to cleanse men from vanity is Your Holy One, in Him is my substance: for from Him are all, whom You have not made for nought, purified from their own vanity. To them it is said, “O you sons of men, how long are you heavy in heart?
Wherefore have ye such pleasure in vanity, and seek after leasing?” Perhaps they might become anxious, and turn from their vanity, and when they found themselves polluted with it, might seek for purification from it: then help them, make them secure. “Know this also, that the Lord has made wonderful His Holy One.” He has made His Holy One to be admired: thence He has purified all from their vanity: there, says David, is my substance: O remember it! “For You have not made all the sons of men for nought.”
You have therefore reserved something to purify them: and who is He whom You have reserved? “What man is he that lives, and shall not see death?” This man then who shall live and not see death, shall purify them from nothingness. For He made not all men for nought, nor can He who made them so despise His own creatures, as not to convert and purify them.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)