How can Jesus be both priest and victim?
Jesus Christ is always the principal and sovereign Priest. From Him, the Apostles and their successors in the priesthood receive the power to celebrate the Eucharistic sacrifice in His name and on behalf of the entire Church. Therefore, following Christ’s command, the priest offers the Mass as the representative of Christ. That is why he does not say, “This is the body and blood of Christ,” but rather, “This is my body” and “This is my blood.” The priest is the chosen instrument of Christ in the same way that a brush is the painter’s tool.
In the Mass, Christ is no longer alone on the cross. Like any other sacrament, the Mass is an action of both Christ and the Church. The Church does not offer a sacrifice different from that of Christ. At the moment of the offertory, the entire Church, hierarchically structured, presents herself for sacrifice alongside Christ. Christ is the only Priest and Victim, and the entire Church participates in this dual role.
Thus, the sacrifice of the Mass is an act of the whole Christ—Head and members. On the part of Jesus, the surrender of self is real and perfect. It is also real for those who are in a state of grace and are united with Christ by charity. For the individual Christian, the surrender of self is genuine to the extent that he shares the dispositions of heart—absolute submission to the will of God—that are found in Jesus’ heart. Sin and attachment to sin are obstacles to sanctity. At Mass, we should profess our desire to struggle to overcome these obstacles.
When the faithful are said to offer the Mass together with the priest, this does not mean that all members of the Church celebrate the Mass in the same way as the priest. Only the celebrant, who alone possesses the ministerial priesthood, performs this role. He has been divinely appointed for this purpose through the sacrament of Holy Orders.
However, “the priest cannot consider himself a ‘proprietor’ who can make free use of the liturgical text and of the sacred rite as if it were his property, stamping it with his own arbitrary personal style. At times, this approach might seem more effective and may better correspond to subjective piety; nevertheless, objectively, it is always a betrayal of the union that should find its proper expression in the sacrament of unity.
“Every priest who offers the holy sacrifice should recall that during this sacrifice, it is not only he and his community that are praying but the whole Church, which expresses her spiritual unity in this sacrament, among other ways, by using the correct liturgical text.”
The faithful are said to offer the Mass with the priest when they unite their praise, petition, expiation, and thanksgiving with the prayer of the priest—indeed, with the prayer of Christ Himself. In doing so, the faithful exercise the element of Christ’s priesthood that is imparted to them at baptism. This participation in Christ’s priesthood is called the 'common priesthood.' All these intentions are presented to God the Father through the priest’s external rite.