5 With all this, you venture to denounce the sacrifices of the Old Testament, and to call them idolatry, and to attribute to us the same impious notion. To answer for ourselves in the first place, while we consider it no longer a duty to offer sacrifices, we recognize sacrifices as part of the mysteries of Revelation, by which the things prophesied were foreshadowed. For they were our examples, and in many and various ways they all pointed to the one sacrifice which we now commemorate.
Now that this sacrifice has been revealed, and has been offered in due time, sacrifice is no longer binding as an act of worship, while it retains its symbolic authority. For these things "were written for our learning, upon whom the end of the world has come." What you object to in sacrifice is the slaughter of animals, though the whole animal creation is intended conditionally in some way for the use of man. You are merciful to beasts, believing them to contain the souls of human beings, while you refuse a piece of bread to a hungry beggar.
The Lord Jesus, on the other hand, was cruel to the swine when He granted the request of the devils to be allowed to enter into them. The same Lord Jesus, before the sacrifice of His passion, said to a leper whom He had cured, "Go, show yourself to the priest, and give the offering, as Moses commanded, for a testimony unto them." When God, by the prophets, repeatedly declares that He needs no offering, as indeed reason teaches us that offerings cannot be needed by Him who stands in need of nothing, the human mind is led to inquire what God wished to teach us by these sacrifices.
For, assuredly, He would not have required offerings of which He had no need, except to teach us something that it would profit us to know, and which was suitably set forth by means of these symbols. How much better and more honorable it would be for you to be still bound by these sacrifices, which have an instructive meaning, though they are not now necessary, than to require your followers to offer to you as food what you believe to be living victims. The Apostle Paul says most appropriately of some who preached the gospel to gratify their appetite, that their "god was their belly." But the arrogance of your impiety goes much beyond this; for, instead of making your belly your god, you do what is far worse in making your belly the purifier of God.
Surely it is great madness to make a pretence of piety in not slaughtering animals, while you hold that the souls of animals inhabit all the food you eat, and yet make what you call living creatures suffer such torture from your hands and teeth.
Source: Reply to Faustus the Manichaean (New Advent)