The Church can be defined as the union of the faithful in the whole world. It is a community of believers forming the Kingdom of God on earth, and also the Mystical Body of Christ. The Greek word έκκλησία (ékklesía) meaning “Church” can be translated as “those who are called.” That makes reference to the call of God to all men: “Repent and believe in the Gospel.” It is remarkable that none of those definitions make allowance for a division. That is because the Church must reflect the unity of God in the Holy Trinity.
John 17, 20-23 — ‘I ask not only on behalf of these, but also on behalf of those who will believe in me through their word, that they may all be one. As You, Father, are in me and I am in You, may they also be in us,[f] so that the world may believe that You have sent me. The glory that You have given me I have given them, so that they may be one, as we are one, I in them and You in me, that they may become completely one, so that the world may know that You have sent me and have loved them even as You have loved me.
Since the Council of Trent, the Catholic Church is defined as the union of all those who profess the same faith in Christ, participating in the Sacraments in obedience to their Bishops, particularly one Bishop representing Christ on Earth, the Bishop of Rome, the Pope.
The definition of Trent excludes apostates, heretics, and schismatics who refuse to submit to the authority of the Pope in union with his Bishops in the whole world. The Second Vatican Council recognized the perfection of this reality for those in full union with the Catholic Church. At the same time the council indicated that — in a less perfect way — the definition includes all those who are baptized, professing faith in Jesus Christ and therefore “belonging to the Lord.”
However, Christ did not create a “Christian denomination.” The multiple Christian groups we see today in the world cannot be considered as a whole reflecting the unity of God in the Holy Trinity; the Kingdom of God which is one and final; or the body of Christ that cannot exist divided.
The Catholic Church teaches, governs, and sanctifies by means of the authority bestowed on her Bishops by Christ Himself. Using that gift for the salvation of all human beings is the primary mission of the Church. Those who reject the authority of the Catholic Church do so at their own peril, placing their eternal salvation in mortal danger.
The authority of the Church comes from Christ
The Gospels show that Christ created his Church to give witness of Himself and His teachings to all mankind. He promised to assist and guide his Church for all time, providing what the Church needs to fulfill her mission. He also taught that commitment to the task at hand must be complete. The work Christ is giving the Church to do is the most important ever and cannot be delayed. Just as Jesus gave all He had for the advance of the Gospel, even to the point of sacrificing His earthly life, we as a Church are expected to do the same.
Luke 9, 57-62 — As they were going along the road, a man said to him, “I will follow you wherever you go.” And Jesus said to him, “Foxes have holes, and birds of the air have nests; but the Son of man has nowhere to lay his head.” To another he said, “Follow me.” But he said, “Lord, let me first go and bury my father.” But he said to him, “Leave the dead to bury their own dead; but as for you, go and proclaim the kingdom of God.” Another said, “I will follow you, Lord; but let me first say farewell to those at my home.” Jesus said to him, “No one who puts his hand to the plow and looks back is fit for the kingdom of God.”
In the first part of this instruction, Jesus talks about the apostolic vocation, indicating how important is to be fully devoted to the proclamation of the Gospel without any reservations. In doing so Jesus manifests His godly authority to demand full dedication from the men He selects for the mission.
Luke 10, 1-9 — After this the Lord appointed seventy others, and sent them on ahead of him, two by two, into every town and place where he himself was about to come. And he said to them, “The harvest is plentiful, but the laborers are few; pray therefore the Lord of the harvest to send out laborers into his harvest. Go your way; behold, I send you out as lambs in the midst of wolves. Carry no purse, no bag, no sandals; and salute no one on the road. Whatever house you enter, first say, ‘Peace be to this house!’ And if a son of peace is there, your peace shall rest upon him; but if not, it shall return to you. And remain in the same house, eating and drinking what they provide, for the laborer deserves his wages; do not go from house to house. Whenever you enter a town and they receive you, eat what is set before you; heal the sick in it and say to them, ‘The kingdom of God has come near to you.’”
Later Jesus commissions seventy men to give witness to the people of Judea. Those men can be considered a prophetic type of the Church, sent to the world with the mission to declare the Gospel.
Luke 10, 10-16 — But whenever you enter a town and they do not receive you, go into its streets and say, ‘Even the dust of your town that clings to our feet, we wipe off against you; nevertheless know this, that the kingdom of God has come near.’ I tell you, it shall be more tolerable on that day for Sodom than for that town. “Woe to you, Chorazin! woe to you, Bethsaida! for if the mighty works done in you had been done in Tyre and Sidon, they would have repented long ago, sitting in sackcloth and ashes. But it shall be more tolerable in the judgment for Tyre and Sidon than for you. And you, Capernaum, will you be exalted to heaven? You shall be brought down to Hades.”He who hears you hears Me, and he who rejects you rejects Me, and he who rejects me rejects Him Who sent Me.”
Jesus concludes with a severe warning about the consequences of rejecting His message: He who hears you hears me, and he who rejects you rejects me, and he who rejects me rejects Him Who sent me.” That is in essence how the Church receives her authority from Christ Himself. According to the Scriptures the authority of the Church is of divine origin. Here is Christ praying to God the Father:
John 17, 17-19 — Sanctify them by the truth; Your word is truth. As You sent Me into the world, I have also sent them into the world. For them I sanctify Myself, so that they too may be sanctified by the truth.
Christ made the Church mater et magistra, mother and teacher. As a mother she has the authority and the responsibility to nurture and guide the flock of Christ.
Matthew 28, 18-20 — And Jesus came and said to them, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you; and lo, I am with you always, to the close of the age.”
As a teacher the Church has the duty to teach the truth of Christ to those who seek to know and obey Him. The Church governs and sanctifies with the authority given to her by Christ.
Acts 1, 12-26 — Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a sabbath day’s journey away; and when they had entered, they went up to the upper room, where they were staying, Peter and John and James and Andrew, Philip and Thomas, Bartholomew and Matthew, James the son of Alphaeus and Simon the Zealot and Judas the son of James. All these with one accord devoted themselves to prayer, together with the women and Mary the mother of Jesus, and with his brethren.
In those days Peter stood up among the brethren (the company of persons was in all about a hundred and twenty), and said, “Brethren, the scripture had to be fulfilled, which the Holy Spirit spoke beforehand by the mouth of David, concerning Judas who was guide to those who arrested Jesus. For he was numbered among us, and was allotted his share in this ministry. (Now this man bought a field with the reward of his wickedness; and falling headlong he burst open in the middle and all his bowels gushed out. And it became known to all the inhabitants of Jerusalem, so that the field was called in their language Akeldama, that is, Field of Blood.) For it is written in the book of Psalms: ‘Let his habitation become desolate, and let there be no one to live in it’; and ‘His office let another take.’
So one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us, beginning from the baptism of John until the day when he was taken up from us—one of these men must become with us a witness to his resurrection.” And they put forward two, Joseph called Barsabbas, who was surnamed Justus, and Matthias. And they prayed and said, “Lord, who knowest the hearts of all men, show which one of these two thou hast chosen to take the place in this ministry and apostleship from which Judas turned aside, to go to his own place.” And they cast lots for them, and the lot fell on Matthias; and he was enrolled with the eleven apostles.
Using the authority given to them, the Apostles appointed successors to continue the mission initiated by Christ. Immediately after the Ascension they gathered at the Cenacle in Jerusalem with Peter leading them. Mary of Nazareth, mother of Jesus is also present. The first order of the day was to select a man to replace Judas Iscariot so that the Apostolic Body of the Church would be complete. Peter and the other Apostles cast lots over two candidates, using a form of consultation common among the Levite priests (see Samuel 30, 7-8) and Matthias was selected to occupy the post of the Iscariot. Notice that both candidates are chosen because they were witnesses of Christ’s resurrection. In this passage it is also very clear that Peter is taking the initiative and leading the apostles through the process of electing the replacement of Judas Iscariot. 
The Bishops of the Catholic Church continue exercising that authority inherited from the Apostles to this day. The hierarchy of the Church subsists until the day of Christ’s return in glory.
The Church has authority to teach doctrine
Christ promised to remain with his Church for ever, guiding her through the presence and action of the Holy Spirit. He prays before the Father for the faith of Peter and his successors.
Luke 22, 32 —But I have prayed for you, Simon [Peter], that your faith may not fail. And when you have turned back, confirm your brothers.”
The power of Christ prayer is perfect and its effectiveness is guaranteed in Scripture:
James 5, 16 — […] The prayer of a righteous person is powerful and effective.
Since no one is more perfectly righteous than Jesus, we can be assured that the Church lead by Peter and his successors will not fail to teach the truth. The very presence of Christ among us guarantees that the Church will accomplish her mission.
Matthew 28, 20 — “Behold, I am with you always, even to the end of the age.”
Matthew 16, 18 — “Upon this rock, I will build My Church, and the powers of Hell shall not prevail against it.”
It is impossible for the Church to teach error and mislead people. The official teachings of the Church are infallible but not the mere opinions of its members. Christ is teaching when the Bishops of the Church teach in unity about matters of faith, morals, and worship.
In the early days of the Church the Apostles had to decide whether pagan converts were obliged to follow the Old Testament laws of circumcision.
Acts 15, 28-29 — “For it has seemed good to the Holy Spirit and to us to lay upon you no greater burden than these necessary things: that you abstain from what has been sacrificed to idols and from blood and from what is strangled and from unchastity. If you keep yourselves from these, you will do well. Farewell.”
The Apostles are exercising their magisterial authority to define the doctrine of the Church. In doing so they are overriding the existing Scripture and nothing less than the Torah! They are not simply doing what they see fit but they are aware of the Holy Spirit guiding them. God is deciding the matter through the Bishops of the Church.
If the Apostles did not have the authority bestowed upon them by Christ, and the assistance of the Holy Spirit, this passage would be proof of the most severe apostasy! After this first example of the magisterial authority of the Apostles, others like Saint Paul, Saint Peter, Saint James, Saint Jude Thaddeus, and Saint John left us many letters in which they exercise their teaching authority as well. The Scriptures leave no doubt that the Apostles, assisted by the Holy Spirit had the power to lead the Church to the revealed truth even to the point of nullifying certain Old Testament commandments and customs.
Next article: Authority of the Church. 2
 New Vademecum of Catholic Apologetics; chapter 4, “The Papacy of Peter” publ. First Light, 2016; by Carlos Caso-Rosendi. To be published — Deo volente — October 2016.