CATECHISM OF THE CATHOLIC CHURCH
SECOND EDITION

Paragraphs 774–777

774 The Greek word mysterion was translated into Latin by two terms: mysterium 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." 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." 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"; 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." 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."
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.