Objection 1. It would seem that Christ Himself was not both priest and victim. For it is the duty of the priest to slay the victim. But Christ did not kill Himself. Therefore He was not both priest and victim.
Objection 2. Further, the priesthood of Christ has a greater similarity to the Jewish priesthood, instituted by God, than to the priesthood of the Gentiles, by which the demons were worshiped. Now in the old Law man was never offered up in sacrifice: whereas this was very much to be reprehended in the sacrifices of the Gentiles, according to Psalm 105:38: "They shed innocent blood; the blood of their sons and of their daughters, which they sacrificed to the idols of Chanaan." Therefore in Christ's priesthood the Man Christ should not have been the victim.
Objection 3. Further, every victim, through being offered to God, is consecrated to God. But the humanity of Christ was from the beginning consecrated and united to God. Therefore it cannot be said fittingly that Christ as man was a victim.
On the contrary, The Apostle says (Ephesians 5:2): "Christ hath loved us, and hath delivered Himself for us, an oblation and a victim [Douay: 'sacrifice'] to God for an odor of sweetness."
I answer that, As Augustine says (De Civ. Dei x, 5): "Every visible sacrifice is a sacrament, that is a sacred sign, of the invisible sacrifice." Now the invisible sacrifice is that by which a man offers his spirit to God, according to Psalm 50:19: "A sacrifice to God is an afflicted spirit." Wherefore, whatever is offered to God in order to raise man's spirit to Him, may be called a sacrifice.
Now man is required to offer sacrifice for three reasons. First, for the remission of sin, by which he is turned away from God. Hence the Apostle says (Hebrews 5:1) that it appertains to the priest "to offer gifts and sacrifices for sins." Secondly, that man may be preserved in a state of grace, by ever adhering to God, wherein his peace and salvation consist. Wherefore under the old Law the sacrifice of peace-offerings was offered up for the salvation of the offerers, as is prescribed in the third chapter of Leviticus. Thirdly, in order that the spirit of man be perfectly united to God: which will be most perfectly realized in glory. Hence, under the Old Law, the holocaust was offered, so called because the victim was wholly burnt, as we read in the first chapter of Leviticus.
Now these effects were conferred on us by the humanity of Christ. For, in the first place, our sins were blotted out, according to Romans 4:25: "Who was delivered up for our sins." Secondly, through Him we received the grace of salvation, according to Hebrews 5:9: "He became to all that obey Him the cause of eternal salvation." Thirdly, through Him we have acquired the perfection of glory, according to Hebrews 10:19: "We have [Vulgate: 'Having'] a confidence in the entering into the Holies" (i.e. the heavenly glory) "through His Blood." Therefore Christ Himself, as man, was not only priest, but also a perfect victim, being at the same time victim for sin, victim for a peace-offering, and a holocaust.
Reply to Objection 1. Christ did not slay Himself, but of His own free-will He exposed Himself to death, according to Isaiah 53:7: "He was offered because it was His own will." Thus He is said to have offered Himself.
Reply to Objection 2. The slaying of the Man Christ may be referred to a twofold will. First, to the will of those who slew Him: and in this respect He was not a victim: for the slayers of Christ are not accounted as offering a sacrifice to God, but as guilty of a great crime: a similitude of which was borne by the wicked sacrifices of the Gentiles, in which they offered up men to idols. Secondly, the slaying of Christ may be considered in reference to the will of the Sufferer, Who freely offered Himself to suffering. In this respect He is a victim, and in this He differs from the sacrifices of the Gentiles. (The reply to the third objection is wanting in the original manuscripts, but it may be gathered from the above.--Ed.) [Some editions, however, give the following reply:
Reply to Objection 3. The fact that Christ's manhood was holy from its beginning does not prevent that same manhood, when it was offered to God in the Passion, being sanctified in a new way--namely, as a victim actually offered then. For it acquired then the actual holiness of a victim, from the charity which it had from the beginning, and from the grace of union sanctifying it absolutely.]
Source: Summa Theologica (New Advent)