Christ truly acts as High Priest and Victim on the Cross, and fulfills the sacrifices of the Old Testament. He is the one High Priest who offers the one, final sacrifice–yet He chooses to act through the priests of His Church to bestow the graces of His one Sacrifice upon us. How is this possible?
Christ as High Priest and Victim
First of all, Christ is both the High Priest and the Victim—both the one offering a sacrifice (the role of a priest) for sins as well as the one sacrifice being offered (since He offered up Himself). Though this cross was an unjust capital punishment, it was a punishment that He willfully submitted to for our sake, to offer Himself up as a sacrifice for our sins. Though the soldiers were the ones who harmed Him and put Him on the cross to die, it was He who willfully chose to submit (being God, He could have chosen to not undergo this, to come down from the cross, etc), and thus He chose to offer Himself up (both internally and externally).
This external and internal sacrifice, unlike those of the Old Testament, was complete and perfect. He was not only the perfect, sinless man (being man and God), thus being the most pure offering; He also offered a complete and total sacrifice since He was not merely offering something important to Him, as those of the Old Testament who offered the first-fruits of their harvest etc.—He offered His very self. In this way, the internal and external sacrifice was completely aligned, was totally complete.
The One, Final Sacrifice
Christ’s sacrifice on the cross is the total and final sacrifice for sins—the fulfillment of the Old Testament sacrifices which were a “type” of Christ’s Sacrifice to come. They, though limited, had effect in so far as they foreshadowed and in a sense participated in the One Sacrifice of Christ.
Christ, being both God and man, was the only one able to make that complete and total sacrifice in atonement for sins; this sacrifice had to be done by a man, because it was man who had sinned against God and had to make atonement—yet it had to be done by God, because the offense against an infinite God (infinitely loving and deserving of love) would require an equal sacrifice in recompense. Thus, the final sacrifice in atonement for sins would have to be made by Christ, true God and true man.
The One Sacrifice Present in the Mass
This one sacrifice of Christ is made present to us at the Holy Sacrifice of the Mass—in the Eucharist where Christ’s body, blood, soul, and divinity is truly and literally made present to us. This is not another sacrifice or a re-sacrificing of Christ—this is making present to us the one sacrifice of Christ and the graces of that sacrifice.
Christ the High Priest Acting Through His Church
Furthermore, the Catholic priest is not taking the place of Christ the High Priest, but rather acts under Christ’s authority and is only a priest in so far as Christ acts through him. The priest who consecrates the bread and wine is not offering this sacrifice on his own authority or in his own right—he is acting in persona Christi, in the person of Christ. This does not mean that he replaces Christ, but rather that Christ acts through Him. In this way, Christ the one High Priest is the one who offers the sacrifice of the Mass (through the priest) and offers Himself to us in the Eucharist.
We as Catholics also believe that we are called to the priesthood of all believers, by which we mean that all Catholics are called to spiritually offer the sacrifice of their prayers and their hearts in union with Christ’s one sacrifice. Both those called to the ministerial priesthood (those ordained by holy orders) and all of us who are called to the priesthood of all believers are only priests in so far as we participate in the one priesthood of Christ—or, one might say, we are only priests (to a greater or lesser degree depending on our calling) in so far as Christ the High Priest acts through us. In this way, we the faithful may spiritually unite our sufferings, our hopes, our joys, our hearts, our whole lives to the one offering of Christ. It is only by this union with the one sacrifice of Christ that our sacrifices have meaning and purpose. This is why we offer up spiritual sacrifices in prayer, and this is especially important and powerful to do during the Sacrifice of the Mass.
Those ordained as priests participate in this priesthood of all believers as well, but Christ also particularly acts through them in offering the Sacrifice of the Mass, as well as acting through them in the Sacrament of Confession (it is not the priest who forgives our sins, but Christ who acts through him) and in the other Sacramants.
Thus, when we go to the Sacrament of Reconciliation, we are encountering Jesus. When we leave the confessional, we hear with the ears of faith that it is Jesus who says to us: “Your sins are forgiven–go in peace.” And when we see the priest consecrate the bread and wine at Mass, we see with the eyes of faith that this is Jesus Himself, offering Himself to us, saying: “This is my Body, which is given up for you.”