CATECHISM OF THE CATHOLIC CHURCH
SECOND EDITION

Paragraphs 810–813

810 "Hence the universal Church is seen to be 'a people brought into unity from the unity of the Father, the Son, and the Holy Spirit'" (LG 4 citing St. Cyprian, De Dom. orat 23: PL 4, 553).
811 "This is the sole Church of Christ, which in the Creed we profess to be one, holy, catholic and apostolic." These four characteristics, inseparably linked with each other, indicate essential features of the Church and her mission. The Church does not possess them of herself; it is Christ who, through the Holy Spirit, makes his Church one, holy, catholic, and apostolic, and it is he who calls her to realize each of these qualities.
812 Only faith can recognize that the Church possesses these properties from her divine source. But their historical manifestations are signs that also speak clearly to human reason. As the First Vatican Council noted, the "Church herself, with her marvelous propagation, eminent holiness, and inexhaustible fruitfulness in everything good, her catholic unity and invincible stability, is a great and perpetual motive of credibility and an irrefutable witness of her divine mission."
813 The Church is one because of her source: "the highest exemplar and source of this mystery is the unity, in the Trinity of Persons, of one God, the Father and the Son in the Holy Spirit." The Church is one because of her founder: for "the Word made flesh, the prince of peace, reconciled all men to God by the cross, . . . restoring the unity of all in one people and one body." The Church is one because of her "soul": "It is the Holy Spirit, dwelling in those who believe and pervading and ruling over the entire Church, who brings about that wonderful communion of the faithful and joins them together so intimately in Christ that he is the principle of the Church's unity." Unity is of the essence of the Church:

What an astonishing mystery! There is one Father of the universe, one Logos of the universe, and also one Holy Spirit, everywhere one and the same; there is also one virgin become mother, and I should like to call her "Church."