As part of our mission, we look to the Compendium for the explanation of why;
The Church is One, Holy, Catholic and Apostolic
The Church has used a "teaching handbook", if you will, since its earliest conception.The Didache (Διδαχ?, Koine Greekfor "Teaching" was such a "teaching handbook" for the earliest Christians (c. 50–160). Now the Church uses the Catechism of the Catholic Church. The Church recently published the Compendium to the Catechism. The Holy Father, Pope Benedict XVI states, "The Comendium which I now present to the Universal Church, is a faithful and sure synthesis of the Catechism of the Catholic Church. I entrust the Compendium above all to the entire Church and, in particular, to every Christian, in order that it may awaken in the Church of the third millennium renewed zeal for evangelization and education in the faith.. With its brevity, clarity and comprehensiveness, is directed to every human being, who, in a world of distractions and multifarious messages, desires to know the Way of Life, the Truth, entrusted by God to His Son's Church!" Compendium - Catechism of the Catholic Church
161. Why is the Church One? The Church is one because she has as her source and exemplar the unity of the Trinity of Persons in one God. As her Founder, and Head, Jesus Christ re-established the unity of all people in one body. As her soul, the Holy Spirit unites all the faithful in communion with Christ. The Church has but one faith, one sacramental life, one apostolic succession, one common hope, and one and the same charity.
162. Where does the one Church of Christ subsist? The one Church of Christ, as a society constituted and organized in the world, subsist in the Catholic Church, governed by the Successor of Peter and the bishops in communion with him. Only through this Church can one obtain the fullness of the means of salvation since the Lord has entrusted all the blessings of the New Covenant to the apostolic college alone whose head is Peter.
Matthew 16:18 "Thou art Peter and upon this Rock I will build my Church and the gates of hell will not prevail." This apostolic succession began with Peter and has never been broken in over 2000 years and is also is infallible in her teaching of the faith because it is the promise of Jesus that it would be guided and protected by the Holy Spirit.
163. How are non-Catholic Christians to be considered? In the churches and ecclesial communities which are separated from full communion with the Catholic Church, many elements of sanctification and truth can be found. All of these blessings come from Christ and lead to Catholic unity. Members of these churches and communities are incorporated into Christ by Baptism and so we recognize them as brothers.
164. How does one commit oneself to work for the unity of Christians? The desire to restore the unit of all Christians is a gift from Christ and a call of the Spirit. This desire involves the entire Church and it is pursued by conversion of the heart, prayer, fraternal knowledge of each other and theological dialog.
165. In what way is the Church Holy? The Church is holy insofar as the Most Holy God is her author. Christ has given himself for her to sanctify her and make her a source of sanctification. The Holy Spirit gives her life with charity. In the Church one finds the fullness of the means of salvation. Holiness is the vocation of each of her members and the purpose of all her activities. The Church counts among her members the Virgin Mary and numerous Saints who are her models and intercessors. The holiness of the Church is the fountain of sanctification of her children who here on earth recognized themselves as sinners ever in need of conversion and purification.
166. Why is the Church called " Catholic"? The Church is Catholic, that is universal, insofar as Christ is present in her: "Where there is Christ Jesus, there is the Catholic Church" (Saint Ignatius of Antioch). The Church proclaims the fullness and the totality of the faith; she bears and administers the fullness of the means of salvation; she is sent out by Christ on a mission to the whole of the human race.
167. Is the particular Church Catholic? Every Particular Church (that is a diocese or eparchy) is Catholic. It is formed by a community of Christians who are in communion of faith and of the sacraments both with their bishops, who is ordained in apostolic succession, and with the Church of Rome which "presides in charity" (Saint Ignatius of Antioch).
168. Who belongs to the Catholic Church? All human beings in various ways belong to or are ordered to the Catholic unity of the people of God. Fully incorporated into the Catholic Church are those who, possessing the Spirit of Christ, are joined to the Church by the bonds of the profession of faith, the sacraments, ecclesiastical government and communion. The baptized who do not enjoy full Catholic unity are in a certain, although imperfect, communion with the Catholic Church.
170. What is the bond that exists between the Catholic Church and non-Christian religions? There is a bond between all peoples which comes especially from the common origin and end of the entire human race. The Catholic Church recognizes that whatever is good or true in other religions comes from God and is a reflection of his truth. As such it can prepare for the acceptance of the Gospel and act as a stimulus toward the unity of humanity in the Church of Christ.
174. Why is the Church Apostolic? The Church is a apostolic in her origin because she has been built on "the foundation of the apostles" ( Ephesians 2:20). She is apostolic in her teaching which is the same as that of the Apostles. She is apostolic by reason of her structure insofar as she is taught, sanctified, and guided until Christ returns by the Apostles through their successors who are the bishops in communion with the successor of Peter.
175. In what does the mission of the Apostles consist? The Word "Apostle" means one who is sent." Jesus, the One sent by the Father, called to himself twelve of his disciples and appointed them as his Apostles, making them the chosen witnesses of his Resurrection and the foundation of his Church. He gave them the command to continue his own mission saying, "As the Father has sent, even so I send you" (John 20:21); and he promised to remain with them until the end of the world.