Catechism of the Catholic Church
774 The Greek word mysterion was translated into Latin by two terms: mystenum and sacramentum. In later usage the term sacramentum emphasizes the visible sign of the hidden reality of salvation which was indicated by the term mysterium. In this sense, Christ himself is the mystery of salvation: "For there is no other mystery of God, except Christ." 196 The saving work of his holy and sanctifying humanity is the sacrament of salvation, which is revealed and active in the Church's sacraments (which the Eastern Churches also call "the holy mysteries"). The seven sacraments are the signs and instruments by which the Holy Spirit spreads the grace of Christ the head throughout the Church which is his Body. The Church, then, both contains and communicates the invisible grace she signifies. It is in this analogical sense, that the Church is called a "sacrament."
775 "The Church, in Christ, is like a sacrament - a sign and instrument, that is, of communion with God and of unity among all men." 197 The Church's first purpose is to be the sacrament of the inner union of men with God. Because men's communion with one another is rooted in that union with God, the Church is also the sacrament of the unity of the human race. In her, this unity is already begun, since she gathers men "from every nation, from all tribes and peoples and tongues"; 198 at the same time, the Church is the "sign and instrument" of the full realization of the unity yet to come.
776 As sacrament, the Church is Christ's instrument. "She is taken up by him also as the instrument for the salvation of all," "the universal sacrament of salvation," by which Christ is "at once manifesting and actualizing the mystery of God's love for men." 199 The Church "is the visible plan of God's love for humanity," because God desires "that the whole human race may become one People of God, form one Body of Christ, and be built up into one temple of the Holy Spirit." 200
777 The word "Church" means "convocation." It designates the assembly of those whom God's Word "convokes," i.e., gathers together to form the People of God, and who themselves, nourished with the Body of Christ, become the Body of Christ.
778 The Church is both the means and the goal of God's plan: prefigured in creation, prepared for in the Old Covenant, founded by the words and actions of Jesus Christ, fulfilled by his redeeming cross and his Resurrection, the Church has been manifested as the mystery of salvation by the outpouring of the Holy Spirit. She will be perfected in the glory of heaven as the assembly of all the redeemed of the earth (cf. Rev 14:4).
779 The Church is both visible and spiritual, a hierarchical society and the Mystical Body of Christ. She is one, yet formed of two components, human and divine. That is her mystery, which only faith can accept.
780 The Church in this world is the sacrament of salvation, the sign and the instrument of the communion of God and men.
English Translation of the Cathechism of the Catholic Church for the United States of America © 1997, United States Catholic Conference, Inc.