THE CELEBRATION OF THE CHRISTIAN MYSTERY
Catechism of the Catholic Church - with paragraph number

Why the liturgy?

1066 In the Symbol of the faith the Church confesses the mystery of the Holy Trinity and of the plan of God's "good pleasure" for all creation: the Father accomplishes the "mystery of his will" by giving his beloved Son and his Holy Spirit for the salvation of the world and for the glory of his name. Such is the mystery of Christ, revealed and fulfilled in history according to the wisely ordered plan that St. Paul calls the "plan of the mystery" and the patristic tradition will call the "economy of the Word incarnate" or the "economy of salvation."

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 rescuing mankind from wrong doing and giving perfect glory to God. He accomplished this work principally by the mystery of his Passion, Resurrection from the dead, and Ascension, whereby dying he destroyed death, rising he restored life. 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.'" For this reason, the Church celebrates in the liturgy above all the 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:

For it is in the liturgy, especially in the divine sacrifice of the Eucharist, that "the work of our rescue 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.

What does the word liturgy mean?

1069 The word "liturgy" originally meant a "public work" or a "service in the name of/on behalf of the people." In Christian tradition it means the participation of the People of God in "the work of God." Through the liturgy Christ, our redeemer and high priest, continues the work of our rescue in, with, and through his Church.
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. 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"; she shares in Christ's priesthood (worship), which is both prophetic (proclamation) and kingly (service of charity).