Summa Theologica I-II, q. 102: Of the Causes of the Ceremonial Precepts
With regard to the uncleanness contracted by those who burnt these sacrifices, the reason is the same as that which we assigned (ad 5) to the sacrifice of the red heifer.
Reply to Objection 7The legal rite did not cleanse the leper of his deformity, but declared him to be cleansed. This is shown by the words of Lev. 14:3, seqq., where it was said that the priest, "when he shall find that the leprosy is cleansed," shall command "him that is to be purified": consequently, the leper was already healed: but he was said to be purified in so far as the verdict of the priest restored him to the society of men and to the worship of God. It happened sometimes, however, that bodily leprosy was miraculously cured by the legal rite, when the priest erred in his judgment.
Now this purification of a leper was twofold: for, in the first place, he was declared to be clean; and, secondly, he was restored, as clean, to the society of men and to the worship of God, to wit, after seven days. At the first purification the leper who sought to be cleansed offered for himself "two living sparrows. . . cedar-wood, and scarlet, and hyssop," in such wise that a sparrow and the hyssop should be tied to the cedar-wood with a scarlet thread, so that the cedar-wood was like the handle of an aspersory: while the hyssop and sparrow were that part of the aspersory which was dipped into the blood of the other sparrow which was "immolated. . . over living waters." These things he offered as an antidote to the four defects of leprosy: for cedar-wood, which is not subject to putrefaction, was offered against the putrefaction; hyssop, which is a sweet-smelling herb, was offered up against the stench; a living sparrow was offered up against numbness; and scarlet, which has a vivid color, was offered up against the repulsive color of leprosy. The living sparrow was let loose to fly away into the plain, because the leper was restored to his former liberty.
On the eighth day he was admitted to divine worship, and was restored to the society of men; but only after having shaved all the hair of his body, and washed his clothes, because leprosy rots the hair, infects the clothes, and gives them an evil smell. Afterwards a sacrifice was offered for his sin, since leprosy was frequently a result of sin: and some of the blood of the sacrifice was put on the tip of the ear of the man that was to be cleansed, "and on the thumb of his right hand, and the great toe of his right foot"; because it is in these parts that leprosy is first diagnosed and felt. In this rite, moreover, three liquids were employed: viz. blood, against the corruption of the blood; oil, to denote the healing of the disease; and living waters, to wash away the filth.
The figurative reason was that the Divine and human natures in Christ were denoted by the two sparrows, one of which, in likeness of His human nature, was offered up in an earthen vessel over living waters, because the waters of Baptism are sanctified by Christ's Passion. The other sparrow, in token of His impassible Godhead, remained living, because the Godhead cannot die: hence it flew away, for the Godhead could not be encompassed by the Passion. Now this living sparrow, together with the cedar-wood and scarlet or cochineal, and hyssop, i.e. faith, hope and charity, as stated above (ad 5), was put into the water for the purpose of sprinkling, because we are baptized in the faith of the God-Man. By the waters of Baptism or of his tears man washes his clothes, i.e. his works, and all his hair, i.e. his thoughts. The tip of the right ear of the man to be cleansed is moistened with some of the blood and oil, in order to strengthen his hearing against harmful words; and the thumb and toe of his right hand and foot are moistened that his deeds may be holy. Other matters pertaining to this purification, or to that also of any other uncleannesses, call for no special remark, beyond what applies to other sacrifices, whether for sins or for trespasses.
Reply to Objections 8 and 9: Just as the people were initiated by circumcision to the divine worship, so were the ministers by some special purification or consecration: wherefore they are commanded to be separated from other men, as being specially deputed, rather than others, to the ministry of the divine worship. And all that was done touching them in their consecration or institution, was with a view to show that they were in possession of a prerogative of purity, power and dignity. Hence three things were done in the institution of ministers: for first, they were purified; secondly, they were adorned and consecrated; thirdly, they were employed in the ministry. All in general used to be purified by washing in water, and by certain sacrifices; but the Levites in particular shaved all the hair of their bodies, as stated in Lev. 8 (cf. Num. 8).
With regard to the high-priests and priests the consecration was performed as follows. First, when they had been washed, they were clothed with certain special garments in designation of their dignity. In particular, the high-priest was anointed on the head with the oil of unction: to denote that the power of consecration was poured forth by him on to others, just as oil flows from the head on to the lower parts of the body; according to Ps. 132:2: "Like the precious ointment on the head that ran down upon the beard, the beard of Aaron." But the Levites received no other consecration besides being offered to the Lord by the children of Israel through the hands of the high-priest, who prayed for them. The lesser priests were consecrated on the hands only, which were to be employed in the sacrifices. The tip of their right ear and the thumb of their right hand, and the great toe of their right foot were tinged with the blood of the sacrificial animal, to denote that they should be obedient to God's law in offering the sacrifices (this is denoted by touching their right ear); and that they should be careful and ready in performing the sacrifices (this is signified by the moistening of the right foot and hand). They themselves and their garments were sprinkled with the blood of the animal that had been sacrificed, in memory of the blood of the lamb by which they had been delivered in Egypt. At their consecration the following sacrifices were offered: a calf, for sin, in memory of Aaron's sin in fashioning the molten calf; a ram, for a holocaust, in memory of the sacrifice of Abraham, whose obedience it behooved the high-priest to imitate; again, a ram of consecration, which was a peace-offering, in memory of the delivery from Egypt through the blood of the lamb; and a basket of bread, in memory of the manna vouchsafed to the people.
In reference to their being destined to the ministry, the fat of the ram, one roll of bread, and the right shoulder were placed on their hands, to show that they received the power of offering these things to the Lord: while the Levites were initiated to the ministry by being brought into the tabernacle of the covenant, as being destined to the ministry touching the vessels of the sanctuary.