The Church is described as having four essential features: one, holy, catholic, and apostolic (see CCC 811 and Lumen Gentium, art. 81). What are these four essential features, and what do they mean?
One: The Church is one because her founder is Christ himself: "There is one body and one Spirit, just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism, one God and Father of us all, who is above all and through all and in all" (Ephesians 4:5). In Christ, we are united by the bonds of unity: our profession of one faith, our common celebration of worship, and apostolic succession (see below for more information) (CCC, art. 815).
Holy: Because Christ alone is all-holy, and Christ is the head of the Church, the Church is therefore also holy. The Second Vatican Council document Lumen Gentium explains this idea well: "The Church on earth is endowed already with a sanctity that is real though imperfect" (art. 48). The Church here on earth is pilgrimaging toward heaven, where she will be made perfect. Therefore, while the Church possesses sanctity here on earth, for she has Christ as her head, she is still on a journey toward perfect sanctity, which will be found in Heaven.
Catholic: The word "catholic" means "universal" (see CCC, art. 830). The Church is universal in two ways. First, Christ is her head, such that his presence also indicates the presence of the Church. Second, the Church is universal in that she is meant to preach the Gospel to all nations: "Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Hoy Spirit" (Matthew 28:19). Christ gave the universal command that all the members of his Body are meant to spread his Gospel throughout the world. Through her internal and external works, the Church reveals to the world the Gospel of Christ, which means that the Church reveals the new law of love.
Apostolic: The Church is apostolic because of the apostles. We can say she is apostolic in three ways: she remains built on the foundation of the apostles through apostolic succcession; through the Holy Spirit, she continues to pass on the faith; and she continue to be sanctified through the successors to the apostles, namely, the priests, the college of bishops, and the Pope as her head (see CCC, art. 857).
St. Paul describes the Church as a body. Bodies have many members, or many different parts, that make up the whole (see 1 Corinthians 12:14-18). Without the veins, the heart cannot function. Without the blood, neither the heart nor the veins can vunction. And without the heart, the whole body cannot live. In a similar way, the Church is comprised of many members, all of whom fulfill their different roles (see 1 Corinthians 12:28-30). The Pope is the visible head, the bishops and the priests are ministers, the consecrated and religious men and women serve the Church through vows of poverty, chastity, and obedience, and the laity fulfill their own mission while pursuing temporal affairs.
Because the Church is the Body of Christ, she exists as communion. There are three types of communion within the Church:
First, as St. Paul explains, "He [Christ] is the head of the body, the Church" (Colossians 1:18). Although he has ascended into Heaven, Christ still rules over his Church, through the Pope, who is the visible head of the Church here on earth. Christ is the one who continues to inspire the Church, for he is the author of her teachings. Christ is the one who acts in the Church, particularly in the sacraments. The Church is nothing without Christ and derives her whole existence from him.
Second, the Church exists in two "phases":
The Church here on earth is imperfect, for while she is governed by Christ, she is ultimately made up of human members, who possess free will and can choose not to follow Christ. While the Church remains in pilgrimage here on earth, offering all her prayers and sacrifices with eyes fixed on the final goal of Heaven, she remains in communion with the saints in Heaven. This is why, after St. Paul describes the faith of the fathers of Israel, he says that "we are surrounded by so great a cloud of witnesses" (Hebrews 12:1). The saints are witnesses to the great glory of Heaven, and as St. Paul continues, "Let us run with perseverance the race that is set before us, looking to Jesus the pioneer and perfecter of our faith, who for the joy that was set before him endured the cross, despising the shame, and is seated at the right hand of the throne of God" (Hebrews 12:2). Our communion with the saints can inspire us to continue moving forward, despite the difficulties of our temporal life.
Finally, in the Church, we have sacramental communion. The seven sacraments (baptism, penance, Eucharist, confirmation, holy orders, marriage, and extreme unction) are given to the Church by Christ. As the Catechism of the Catholic Church explains, "Christ now acts through the sacraments he instituted to communicate his grace. The sacraments are perceptible signs (words and actions) accessible to our human nature" (CCC, art. 1084). Thus, by Christ's action and the power of the Holy Spirit, the sacraments impart to us the grace of Christ, the graces we need in order to live a life in union with his. The sacraments unite the whole Church: "There is one body and one spirit, just as you were called to the one hope that belongs to your call, one Lord, one faith, one baptism, one God and Father of us all" (Ephesians 4:4-5). Through one baptism, we are united in one Eucharist, which is the Body and Blood of Jesus Christ. The whole Church is united through the celebration of the same Eucharist, which is also a sign of how we are united in Christ.
Some people might object to the Catholic Church because of her hierarchical structure. The Church may seem to be only an external institution, rather than an organic, living body. But the hierarchy of the Church is not simply arbitrary, but comes from Christ himself, who established the apostles as the ones who would pass on the Catholic faith. Thus, the Second Vatican II document, Lumen Gentium (the Dogmatic Constitution on the Church), explains, "For the discharging of such great duties [passing on the faith], the apostles were enriched by Christ with a special outpouring of the Holy Spirit coming upon them, and they passed on this spiritual gift to their helpers by the imposition of hands, and it has been transmitted down to us in episcopal consecration" (art. 21). In other words, Christ himself is the one who ordains his ministers through apostolic succession. Thus, the hierarchy of the Church is not imposed or man-made; rather, Christ himself, as the Great High Priest (see Hebrews 4:14), established the hierarchy.
Moreover, there is collegial union between the Pope and the bishops. The Pope is the visible head of the universal Church, while the individual bishops are the visible head of their own dioceses (or particular churches). However, "all of them together and with the Pope represent the entire Church in the bond of peace, love, and unity" (Lumen Gentium, art. 23). For this reason, a "particular church" cannot depart from the universal Church in its own ideas or practices. The particular churches must remain in union with the universal Church, which means they must remain in union with the Chair of Peter, the Pope. While some may object that this unity means conformity, Lumen Gentium explains, "This variety of local churches with one common aspiration is splendid evidence of the catholicity of the undivided Church" (Lumen Gentium, art. 23). While the local churches must remain in unity with the the universal Church, that does not negate "their own discipline, their own liturgical usage, and their own theological spiritual heritage" (Lumen Gentium, art. 23).
The bishops play a very particular role in the Church. They possess the threefold office of sanctifying, teaching, and governing: "Bishops, in an eminent and visible way, sustain the roles of Christ himself as teacher, shepherd, and high priest" (Lumen Gentium, art. 21). Bishops are the image of the Pope for their particular church, and in a special way, the image of Christ. The flock of the church follows the local bishop. He teaches them the way to follow Christ and the way to fulfill their own vocation within the Church, depending on their state in life. He gives them the sacraments, especially the sacrament of the Eucharist, to nourish them and lead them into eternal life. Finally, he governs his diocese, and ensures that all are following the law of the Church and the law of Christ. Thus, the bishop holds a very distingished place in the Church, being a public sign for the faithful of the proper way to know, love, and serve Christ.