Summa Theologica I-II, q. 102: Of the Causes of the Ceremonial Precepts
The third kind of sacrifice was called the "peace-offering," which was offered to God, either in thanksgiving, or for the welfare and prosperity of the offerers, in acknowledgment of benefits already received or yet to be received: and this typifies the state of those who are proficient in the observance of the commandments. These sacrifices were divided into three parts: for one part was burnt in honor of God; another part was allotted to the use of the priests; and the third part to the use of the offerers; in order to signify that man's salvation is from God, by the direction of God's ministers, and through the cooperation of those who are saved.
But it was the universal rule that the blood and fat were not allotted to the use either of the priests or of the offerers: the blood being poured out at the foot of the altar, in honor of God, while the fat was burnt upon the altar (Lev. 9:9,10). The reason for this was, first, in order to prevent idolatry: because idolaters used to drink the blood and eat the fat of the victims, according to Dt. 32:38: "Of whose victims they eat the fat, and drank the wine of their drink-offerings." Secondly, in order to form them to a right way of living. For they were forbidden the use of the blood that they might abhor the shedding of human blood; wherefore it is written (Gn. 9:4,5): "Flesh with blood you shall not eat: for I will require the blood of your lives": and they were forbidden to eat the fat, in order to withdraw them from lasciviousness; hence it is written (Ezech. 34:3): "You have killed that which was fat." Thirdly, on account of the reverence due to God: because blood is most necessary for life, for which reason "life" is said to be "in the blood" (Lev. 17:11, 14): while fat is a sign of abundant nourishment. Wherefore, in order to show that to God we owe both life and a sufficiency of all good things, the blood was poured out, and the fat burnt up in His honor. Fourthly, in order to foreshadow the shedding of Christ's blood, and the abundance of His charity, whereby He offered Himself to God for us.
In the peace-offerings, the breast-bone and the right shoulder were allotted to the use of the priest, in order to prevent a certain kind of divination which is known as "spatulamantia," so called because it was customary in divining to use the shoulder-blade [spatula], and the breast-bone of the animals offered in sacrifice; wherefore these things were taken away from the offerers. This also denoted the priest's need of wisdom in the heart, to instruct the people—this was signified by the breast-bone, which covers the heart; and his need of fortitude, in order to bear with human frailty—and this was signified by the right shoulder.
Reply to Objection 9Because the holocaust was the most perfect kind of sacrifice, therefore none but a male was offered for a holocaust: because the female is an imperfect animal. The offering of turtledoves and doves was on account of the poverty of the offerers, who were unable to offer bigger animals. And since peace-victims were offered freely, and no one was bound to offer them against his will, hence these birds were offered not among the peace-victims, but among the holocausts and victims for sin, which man was obliged to offer at times. Moreover these birds, on account of their lofty flight, were befitting the perfection of the holocausts: and were suitable for sin-offerings because their song is doleful.
Reply to Objection 10The holocaust was the chief of all the sacrifices: because all were burnt in honor of God, and nothing of it was eaten. The second place in holiness, belongs to the sacrifice for sins, which was eaten in the court only, and on the very day of the sacrifice (Lev. 7:6, 15). The third place must be given to the peace-offerings of thanksgiving, which were eaten on the same day, but anywhere in Jerusalem. Fourth in order were the "ex-voto" peace-offerings, the flesh of which could be eaten even on the morrow. The reason for this order is that man is bound to God, chiefly on account of His majesty; secondly, on account of the sins he has committed; thirdly, because of the benefits he has already received from Him; fourthly, by reason of the benefits he hopes to receive from Him.
Reply to Objection 11Sins are more grievous by reason of the state of the sinner, as stated above (Q[73], A[10]): wherefore different victims are commanded to be offered for the sin of a priest, or of a prince, or of some other private individual. "But," as Rabbi Moses says (Doct. Perplex. iii), "we must take note that the more grievous the sin, the lower the species of animals offered for it. Wherefore the goat, which is a very base animal, was offered for idolatry; while a calf was offered for a priest's ignorance, and a ram for the negligence of a prince."
Reply to Objection 12In the matter of sacrifices the Law had in view the poverty of the offerers; so that those who could not have a four-footed animal at their disposal, might at least offer a bird; and that he who could not have a bird might at least offer bread; and that if a man had not even bread he might offer flour or ears of corn.
The figurative cause is that the bread signifies Christ Who is the "living bread" (Jn. 6:41, 51). He was indeed an ear of corn, as it were, during the state of the law of nature, in the faith of the patriarchs; He was like flour in the doctrine of the Law of the prophets; and He was like perfect bread after He had taken human nature; baked in the fire, i.e. formed by the Holy Ghost in the oven of the virginal womb; baked again in a pan by the toils which He suffered in the world; and consumed by fire on the cross as on a gridiron.
Reply to Objection 13The products of the soil are useful to man, either as food, and of these bread was offered; or as drink, and of these wine was offered; or as seasoning, and of these oil and salt were offered; or as healing, and of these they offered incense, which both smells sweetly and binds easily together.
Now the bread foreshadowed the flesh of Christ; and the wine, His blood, whereby we were redeemed; oil betokens the grace of Christ; salt, His knowledge; incense, His prayer.
Reply to Objection 14Honey was not offered in the sacrifices to God, both because it was wont to be offered in the sacrifices to idols; and in order to denote the absence of all carnal sweetness and pleasure from those who intend to sacrifice to God. Leaven was not offered, to denote the exclusion of corruption. Perhaps too, it was wont to be offered in the sacrifices to idols.
Salt, however, was offered, because it wards off the corruption of putrefaction: for sacrifices offered to God should be incorrupt. Moreover, salt signifies the discretion of wisdom, or again, mortification of the flesh.
Incense was offered to denote devotion of the heart, which is necessary in the offerer; and again, to signify the odor of a good name: for incense is composed of matter, both rich and fragrant. And since the sacrifice "of jealousy" did not proceed from devotion, but rather from suspicion, therefore incense was not offered therein (Num. 5:15).