13 “Burnt-offerings also for sin have You not required.” “Then said I, Lo, I come!”. It is time that what “was promised should come;” because the signs, by means of which they were promised, have been put away. And indeed, Brethren, observe these put away; those fulfilled. Let the Jewish nation at this time show me their priest, if they can! Where are their sacrifices? They are brought to an end; they are put away now. Should we at that time have rejected them? We do reject them now; because, if you chose to celebrate them now, it were unseasonable; unfitting at the time; incongruous.
You are still making promises; I have already received! There has remained to them a certain thing for them to celebrate; that they might not remain altogether without a sign....In such a case then are they; like Cain with his mark. The sacrifices, however, which used to be performed there, have been put away; and that which remained unto them for a sign like that of Cain, has by this time been fulfilled; and they know it not. They slay the Lamb; they eat the unleavened bread.
“Christ has been sacrificed for us, as our Passover.” Lo, in the sacrifice of Christ, I recognise the Lamb that was slain! What of the unleavened bread? “Therefore,” says he, “let us keep the feast; not with old leaven, neither with the leaven of wickedness” (he shows what is meant by “old;” it is “stale” flour; it is sour), “but in the unleavened bread of sincerity and truth.” They have continued in the shade; they cannot abide the Sun of Glory. We are already in the light of day. We have “the Body” of Christ, we have the Blood of Christ. If we have a new life, let us “sing a new song, even a hymn unto our God.” “Burnt offerings for sin You did not desire. Then said I, Lo, I come!”
Source: The Enarrations, or Expositions, on the Psalms (New Advent)