12 “Sacrifice and offering You did not desire”, says the Psalm to God. For the men of old time, when as yet the true Sacrifice, which is known to the faithful, was foreshown in figures, used to celebrate rites that were figures of the reality that was to be hereafter; many of them understanding their meaning; but more of them in ignorance of it. For the Prophets and the holy Patriarchs understood what they were celebrating; but the rest of the “stiff-necked people” were so carnal, that what was done by them was but to symbolize the things that were to come afterwards; and it came to pass, when that first sacrifice was abolished; when the burnt-offerings of “rams, of goats, and of calves,” and of other victims, had been abolished, “God did not desire them.” Why did God not desire them? And why did He at the first desire them? Because all those things were, as it were, the words of a person making a promise; and the expressions conveying a promise, when the thing that they promise has come, are no longer uttered....Those sacrifices then, as being but expressions of a promise, have been abrogated. What is that which has been given as its fulfilment? That “Body;” which you know; which you do not all of you know; which, of you who do know it, I pray God all may not know it unto condemnation. Observe the time when it was said; for the person is Christ our Lord, speaking at one time for His members, at another in His own person. “Sacrifice and offering,” said He, “You did not desire.” What then? Are we left at this present time without a sacrifice? God forbid!
“But a Body have You perfected for me.” It was for this reason that You did not desire the others; that You might “perfect” this; before You “perfected” this, You desired the others. The fulfilment of the promise has done away with the words that express the promise. For if they still hold out a promise, that which was promised is not yet fulfilled. This was promised by certain signs; the signs that convey the promise are done away; because the Substance that was promised has come. We are in this “Body.” We are partakers of this “Body.” We know that which we ourselves receive; and you who know it not yet, will know it bye and bye; and when you come to know it, I pray ye may not receive it unto condemnation. “For he that eats and drinks unworthily, eats and drinks damnation unto himself.” “A Body” has been “perfected” for us; let us be made perfect in the Body.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)