CATECHISM OF THE CATHOLIC CHURCH
SECOND EDITION

Paragraphs 747–751

747 The Holy Spirit, whom Christ the head pours out on his members, builds, animates, and sanctifies the Church. She is the sacrament of the Holy Trinity's communion with men.
748 "Christ is the light of humanity; and it is, accordingly, the heart-felt desire of this sacred Council, being gathered together in the Holy Spirit, that, by proclaiming his Gospel to every creature, it may bring to all men that light of Christ which shines out visibly from the Church." These words open the Second Vatican Council's Dogmatic Constitution on the Church. By choosing this starting point, the Council demonstrates that the article of faith about the Church depends entirely on the articles concerning Christ Jesus. The Church has no other light than Christ's; according to a favorite image of the Church Fathers, the Church is like the moon, all its light reflected from the sun.
749 The article concerning the Church also depends entirely on the article about the Holy Spirit, which immediately precedes it. "Indeed, having shown that the Spirit is the source and giver of all holiness, we now confess that it is he who has endowed the Church with holiness." The Church is, in a phrase used by the Fathers, the place "where the Spirit flourishes."
750 To believe that the Church is "holy" and "catholic," and that she is "one" and "apostolic" (as the Nicene Creed adds), is inseparable from belief in God, the Father, the Son, and the Holy Spirit. In the Apostles' Creed we profess "one Holy Church" (Credo . . . Ecclesiam), and not to believe in the Church, so as not to confuse God with his works and to attribute clearly to God's goodness all the gifts he has bestowed on his Church.
751 The word "Church" (Latin ecclesia, from the Greek ek-ka-lein, to "call out of") means a convocation or an assembly. It designates the assemblies of the people, usually for a religious purpose. Ekklesia is used frequently in the Greek Old Testament for the assembly of the Chosen People before God, above all for their assembly on Mount Sinai where Israel received the Law and was established by God as his holy people. By calling itself "Church," the first community of Christian believers recognized itself as heir to that assembly. In the Church, God is "calling together" his people from all the ends of the earth. The equivalent Greek term Kyriake, from which the English word Church and the German Kirche are derived, means "what belongs to the Lord."