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 Ps. 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 (Heb. 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 Rm. 4:25: "Who was delivered up for our sins." Secondly, through Him we received the grace of salvation, according to Heb. 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 Heb. 10:19: "We have [Vulg.: '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.