The Church is Catholic
From the Catechism of the Catholic Church, Simplified
Catholic (Universal) in Means and in Mission (830-831)
The Church is Catholic (meaning "universal") in two ways. First, the Church is universal because Christ is present in her. She receives from him "the fullness of the means of salvation." In this fundamental sense, the Church was Catholic on the day of Pentecost and will be so until Christ returns.
Second, the Church is Catholic because she has a mission to the whole world. The new People of God (while remaining one) must spread throughout the world. God made human nature one and now he decrees that all the scattered people be gathered together. By its gift of universality, the church seeks the return of all humanity under Christ the Head (Second Vatican Council).
Catholic Even When Local (832-833)
The Church is really present in all legitimately organized local groups of the faithful united to their pastors (called "churches" in the New Testament). These communities might be small and poor. Yet, in these particular churches Christ is present and the One, Holy, Catholic, and Apostolic Church is constituted (Second Vatican Council).
The diocese (or eparchy) is a community of faithful headed by a bishop ordained in apostolic succession. These particular churches are modeled after the universal Church. In them the one and unique Catholic Church exists.
Unity in Rome (834)
Particular Churches are fully Catholic by their unity with Rome "which presides in charity" (St. Ignatius of Antioch). "For with this Church (Rome), by reason of its pre-eminence, the whole Church must necessarily be in accord" (St. Irenaeus). "All Christian Churches have held the great Church of Rome as their basis and foundation since, the gates of hell have never prevailed against her" (St. Maximus the Confessor).
In a Variety of Cultures (835)
The universal Church is not just a federation of different particular Churches. The universal Church is rooted in a variety of cultures and takes on different external expressions. The rich variety shows forth the Catholicity of the undivided Church (Pope Paul VI).
All are Called (836)
All men are certainly called to this Catholic unity. The Catholic faithful, others who believe in Christ, and all mankind belong to or are ordered to Catholic unity.
Some are Incorporated (837-838)
The fully incorporated are those who accept all the Church's means of salvation and who, by profession of faith, the sacraments, church government, and communion, are united in the visible structure of the Church. However, a Church member who does not persevere in charity is not saved. He is in the Church's bosom, but "in body" not "in heart" (Second Vatican Council).
Others who are baptized and are called "Christian" but who do not profess the Catholic faith are still joined to the Catholic Church in many ways. They enjoy a certain, although imperfect, communion with the Catholic Church. The unity with the Orthodox Church is so profound that it lacks little to attain the fullness to permit a common celebration of Eucharist (Pope Paul VI).
The Jews - Awaiting an Unknown Messiah (839-840)
Those who have not heard the Gospel are related to the People of God in various ways.
The Jewish People were the first to hear the Word of God, and their faith is already a response to God's revelation. To them "belong the sonship, the glory, the covenants, the giving of the law, the worship and the promises" (Rom 9:4-5) and "this call of God is irrevocable" (Rom 11:29).
The People of God of the Old Covenant and new People of God expect the coming (or the return) of the Messiah. However, the People of the New Covenant await the return of a risen Messiah. The People of the Old Covenant await the coming of a still unknown Messiah, because they do not know or they misunderstand Christ Jesus.
God's plan of salvation includes those who acknowledge the Creator. Among these, in the first place, are the Muslims who profess the faith of Abraham and believe in one merciful God as mankind's judge on the Last Day.
A Common Origin (842)
The Church's bond with non-Christian religions is the common origin and goal of the human race. "All nations stem from one stock and all share a common destiny, namely, God. His providence extends to all" (Second Vatican Council).
Other Religions (843)
The Church recognizes that in other religions there is a search for a God. Any goodness or truth in these religions is a "preparation for the Gospel" (Second Vatican Council).
Possible Mistakes in Religious Behavior (844)
In his religious behavior, man can make mistakes. "Deceived by the Evil One, men have exchanged the truth of God for a lie and served the creature rather than the Creator. Without God, they are exposed to ultimate despair" (Second Vatican Council).
The Church as Noah's Ark (845)
The Father wants to reunite all humanity into his Son's Church. According to St. Augustine and St. Ambrose, the Church was prefigured by Noah's ark, which alone saved the world from the flood.
"Outside the Church There is No Salvation" (846)
How do we understand this saying from the Church Fathers? All salvation comes from Christ through his Body, the Church which is necessary for salvation because Christ is present in his Church.
Jesus said, "The man who believes and accepts Baptism will be saved; the man who refuses to believe in it will be condemned" (Mk 16:16). By these words Jesus also affirmed the necessity of the Church, because Baptism is its door to the Church.
Refusing to Enter (847)
Someone who knows the Catholic Church was founded as necessary by God and refuses to enter or remain in it, cannot be saved.
Just Not Knowing (848)
However, those, who through no fault of their own do not know either the Gospel of Christ or his Church, can achieve salvation by seeking God with a sincere heart and by trying to do God's will (Second Vatican Council). Although God can lead all people to salvation, the Church still has the duty to evangelize all men.