1067 “The wonderful works of God among the people of the Old Testament were but a prelude to the work of Christ the Lord in redeeming mankind and giving perfect glory to God. He accomplished this work principally by the Paschal mystery of his blessed Passion, Resurrection from the dead, and glorious Ascension, whereby ‘dying he destroyed our death, rising he restored our life.’ [Memorial Acclamation B from the Order of the Mass, Ordinary Form] For it was from the side of Christ as he slept the sleep of death upon the cross that there came forth ‘the wondrous sacrament of the whole Church’” (Sacrosanctum Concilium [Vatican II, Constitution on the Liturgy] 5 §2; cf. St. Augustine, Ennarrationes in Psalmos 138, 2). For this reason, the Church celebrates in the liturgy above all the Paschal mystery by which Christ accomplished the work of our salvation.

1068 It is this mystery of Christ that the Church proclaims and celebrates in her liturgy so that the faithful may live from it and bear witness to it in the world:

For it is in the liturgy, especially in the divine sacrifice of the Eucharist [in its identity with the sacrifice of the cross], that “the work of our redemption is accomplished,” and it is through the liturgy especially that the faithful are enabled to express in their lives and manifest to others the mystery of Christ and the real nature of the true Church (Sacrosanctum Concilium 2).

…1070 In the New Testament the word “liturgy” refers not only to the celebration of divine worship but also to the proclamation of the Gospel and to active charity (cf. Lk 1:23; Acts 13:2; Rom 15:16, 27; 2 Cor 9:12; Phil 2:14-17, 25, 30). In all of these situations it is a question of the service of God and neighbor. In a liturgical celebration the Church is servant in the image of her Lord, the one “leitourgos” (cf. Heb 8:2, 6); she shares in Christ’s priesthood (worship), which is both prophetic (proclamation) and kingly (service of charity):

The liturgy then is rightly seen as an exercise of the priestly office of Jesus Christ. It involves the presentation of man’s sanctification under the guise of signs perceptible by the senses and its accomplishment in ways appropriate to each of these signs. In it full public worship is performed by the Mystical Body of Jesus Christ, that is, by the Head and his members. From this it follows that every liturgical celebration, because it is an action of Christ the priest and of his Body which is the Church, is a sacred action surpassing all others. No other action of the Church can equal its efficacy by the same title and to the same degree. (Sacrosanctum Concilium 7 §2–3)

(84) [W]e deem it necessary to recall that the priest acts for the people only because he represents Jesus Christ, who is Head of all His members and offers Himself in their stead. ...

(85) However, it must also be said that the faithful do offer the divine Victim, though in a different sense. ...

(86) "Not only," says Innocent III of immortal memory, "do the priests offer the sacrifice, but also all the faithful: for what the priest does personally by virtue of his ministry, the faithful do collectively by virtue of their intention." We are happy to recall one of St. Robert Bellarmine's many statements on this subject. "The sacrifice," he says "is principally offered in the person of Christ. Thus the oblation that follows the consecration is a sort of attestation that the whole Church consents in the oblation made by Christ, and offers it along with Him."

(87) Moreover, the rites and prayers of the eucharistic sacrifice signify and show no less clearly that the oblation of the Victim is made by the priests in company with the people. For not only does the sacred minister, after the oblation of the bread and wine when he turns to the people, say the significant prayer: "Pray brethren, that my sacrifice and yours may be acceptable to God the Father Almighty;" but also the prayers by which the divine Victim is offered to God are generally expressed in the plural number: and in these it is indicated more than once that the people also participate in this august sacrifice inasmuch as they offer the same. ...

(88) Nor is it to be wondered at, that the faithful should be raised to this dignity. ... [T]hey participate, according to their condition, in the priesthood of Christ.

(89) It is fitting, then, that the Christian people should also desire to know in what sense they are said in the canon of the Mass to offer up the sacrifice. ...

(90) First of all the more extrinsic explanations are these: it frequently happens that the faithful assisting at Mass join their prayers alternately with those of the priest, and sometimes - a more frequent occurrence in ancient times - they offer to the ministers at the altar bread and wine to be changed into the body and blood of Christ, and, finally, by their alms they get the priest to offer the divine victim for their intentions.

(91) But there is also a more profound reason why all Christians, especially those who are present at Mass, are said to offer the sacrifice.

(92) In this most important subject it is necessary, in order to avoid giving rise to a dangerous error, that we define the exact meaning of the word "offer." The unbloody immolation at the words of consecration, when Christ is made present upon the altar in the state of a victim, is performed by the priest and by him alone, as the representative of Christ and not as the representative of the faithful. But it is because the priest places the divine victim upon the altar that he offers it to God the Father as an oblation for the glory of the Blessed Trinity and for the good of the whole Church. Now the faithful participate in the oblation, understood in this limited sense, after their own fashion and in a twofold manner, namely, because they not only offer the sacrifice by the hands of the priest, but also, to a certain extent, in union with him. It is by reason of this participation that the offering made by the people is also included in liturgical worship.

(93) Now it is clear that the faithful offer the sacrifice by the hands of the priest from the fact that the minister at the altar, in offering a sacrifice in the name of all His members, represents Christ, the Head of the Mystical Body. Hence the whole Church can rightly be said to offer up the victim through Christ. But the conclusion that the people offer the sacrifice with the priest himself is not based on the fact that, being members of the Church no less than the priest himself, they perform a visible liturgical rite; for this is the privilege only of the minister who has been divinely appointed to this office: rather it is based on the fact that the people unite their hearts in praise, impetration, expiation and thanksgiving with prayers or intention of the priest, even of the High Priest himself, so that in the one and same offering of the victim and according to a visible sacerdotal rite, they may be presented to God the Father. It is obviously necessary that the external sacrificial rite should, of its very nature, signify the internal worship of the heart. Now the sacrifice of the New Law signifies that supreme worship by which the principal Offerer himself, who is Christ, and, in union with Him and through Him, all the members of the Mystical Body pay God the honor and reverence that are due to Him.