I answer that, In the priestly office, we may consider two things: first, the offering of the sacrifice; secondly, the consummation of the sacrifice, consisting in this, that those for whom the sacrifice is offered, obtain the end of the sacrifice. Now the end of the sacrifice which Christ offered consisted not in temporal but in eternal good, which we obtain through His death, according to Heb. 9:11: "Christ is [Vulg.: 'being come'] a high-priest of the good things to come"; for which reason the priesthood of Christ is said to be eternal. Now this consummation of Christ's sacrifice was foreshadowed in this, that the high-priest of the Old Law, once a year, entered into the Holy of Holies with the blood of a he-goat and a calf, as laid down, Lev. 16:11, and yet he offered up the he-goat and calf not within the Holy of Holies, but without. In like manner Christ entered into the Holy of Holies—that is, into heaven—and prepared the way for us, that we might enter by the virtue of His blood, which He shed for us on earth.
Reply Obj. 1: The Saints who will be in heaven will not need any further expiation by the priesthood of Christ, but having expiated, they will need consummation through Christ Himself, on Whom their glory depends, as is written (Apoc. 21:23): "The glory of God hath enlightened it"—that is, the city of the Saints—"and the Lamb is the lamp thereof."
Reply Obj. 2: Although Christ's passion and death are not to be repeated, yet the virtue of that Victim endures for ever, for, as it is written (Heb. 10:14), "by one oblation He hath perfected for ever them that are sanctified."
Wherefore the reply to the third objection is clear.
As to the unity of this sacrifice, it was foreshadowed in the Law in that, once a year, the high-priest of the Law entered into the Holies, with a solemn oblation of blood, as set down, Lev. 16:11. But the figure fell short of the reality in this, that the victim had not an everlasting virtue, for which reason those sacrifices were renewed every year. _______________________
SIXTH ARTICLE [III, Q. 22, Art. 6]
Whether the Priesthood of Christ Was According to the Order of
Melchisedech?
Objection 1: It would seem that Christ's priesthood was not according to the order of Melchisedech. For Christ is the fountain-head of the entire priesthood, as being the principal priest. Now that which is principal is not secondary in regard to others, but others are secondary in its regard. Therefore Christ should not be called a priest according to the order of Melchisedech.
Obj. 2: Further, the priesthood of the Old Law was more akin to Christ's priesthood than was the priesthood that existed before the Law. But the nearer the sacraments were to Christ, the more clearly they signified Him; as is clear from what we have said in the Second Part (II-II, Q. 2, A. 7). Therefore the priesthood of Christ should be denominated after the priesthood of the Law, rather than after the order of Melchisedech, which was before the Law.
Obj. 3: Further, it is written (Heb. 7:2, 3): "That is 'king of peace,' without father, without mother, without genealogy; having neither beginning of days nor ending of life": which can be referred only to the Son of God. Therefore Christ should not be called a priest according to the order of Melchisedech, as of some one else, but according to His own order.