Catechism of the Council of Trent: The Holy Catholic Church, the communion of saints
Those Who Are Not Members Of The Church
Hence there are but three classes of persons excluded from the Church's pale: infidels, heretics and schismatics, and excommunicated persons. Infidels are outside the Church because they never belonged to, and never knew the Church, and were never made partakers of any of her Sacraments. Heretics and schismatics are excluded from the Church, because they have separated from her and belong to her only as deserters belong to the army from which they have deserted. It is not, however, to be denied that they are still subject to the jurisdiction of the Church, inasmuch as they may be called before her tribunals, punished and anathematised. Finally, excommunicated persons are not members of the Church, because they have been cut off by her sentence from the number of her children and belong not to her communion until they repent.
But with regard to the rest, however wicked and evil they may be, it is certain that they still belong to the Church. Of this the faithful are frequently to be reminded, in order to be convinced that, were even the lives of her ministers debased by crime, they are still within the Church, and therefore lose nothing of their power.
Other Uses of the Word "Church"
Portions of the Universal Church are usually called churches, as when the Apostle mentions the Church at Corinth, at Galatia, of the Laodiceans, of the Thessalonians.
The private families of the faithful he also calls churches. The church in the family of Priscilla and Aquila he commands to be saluted; and in another place, he says: Aquila and Priscilla with the church that is in their house salute you much in the Lord. Writing to Philemon, he makes use of the same word.
Sometimes, also, the word church is used to signify the prelates and pastors of the church. If he will not hear thee, says our Lord, tell the church. Here the word church means the authorities of the Church.
The place in which the faithful assemble to hear the Word of God, or for other religious purposes, is also called a church. But in this Article, the word church is specially used to signify both the good and the bad, the governed, as well as the governing.
The Marks Of The Church
The distinctive marks of the Church are also to be made known to the faithful, that thus they may be enabled to estimate the extent of the blessing conferred by God on those who have had the happiness to be born and educated within her pale.
"One"
The first mark of the true Church is described in the Nicene Creed, and consists in unity: My dove is one, my beautiful one is one. So vast a multitude, scattered far and wide, is called one for the reasons mentioned by St. Paul in his Epistle to the Ephesians: One Lord, one faith, one baptism.
Unity In Government
The Church has but one ruler and one governor, the invisible one, Christ, whom the eternal Father hath made head over all the Church, which is his body; the visible one, the Pope, who, as legitimate successor of Peter, the Prince of the Apostles, fills the Apostolic chair.
It is the unanimous teaching of the Fathers that this visible head is necessary to establish and preserve unity in the Church. This St. Jerome clearly perceived and as clearly expressed when, in his work against Jovinian, he wrote: One is elected that, by the appointment of a head, all occasion of schism may be removed. In his letter to Pope Damasus the same holy Doctor writes: Away with envy, let the ambition of Roman grandeur cease! I speak to the successor of the fisherman, and to the disciple of the cross. Following no chief but Christ, I am united in communion with your Holiness, that is, with the chair of Peter. I know that on that rock is built the Church. Whoever will eat the lamb outside this house is profane; whoever is not in the ark of Noah shall perish in the flood.
The same doctrine was long before established by Saints Irenaeus and Cyprian. The latter, speaking of the unity of the Church observes: The Lord said to Peter, I say to thee, Peter! thou art Peter: and upon this rock I will build my Church. He builds His Church on one. And although after His Resurrection He gave equal power to all His Apostles, saying: As the Father hath sent me, I also send you, receive ye the Holy Ghost; yet to make unity more manifest, He decided by His own authority that it should be derived from one alone, etc.
Again, Optatus of Milevi says: You cannot be excused on the score of ignorance, knowing as you do that in the city of Rome the episcopal chair was first conferred on Peter, who occupied it as head of the Apostles; in order that in that one chair the unity of the Church might be preserved by all, and that the other Apostles might not claim each a chair for himself; so that now he who erects another in opposition to this single chair is a schismatic and a prevaricator.
Later on St. Basil wrote: Peter is made the foundation, because he says: Thou art Christ, the Son of the Living God; and hears in reply that he is a rock. But although a rock, he is not such a rock as Christ; for Christ is truly an immovable rock, but Peter, only by virtue of that rock. For Jesus bestows His dignities on others; He is a priest, and He makes priests; a rock, and He makes a rock; what belongs to Himself, He bestows on His servants.
Lastly, St. Ambrose says: Because he alone of all of them professed (Christ) he was placed above all.
Should anyone object that the Church is content with one Head and one Spouse, Jesus Christ, and requires no other, the answer is obvious. For as we deem Christ not only the author of all the Sacraments, but also their invisible minister—He it is who baptises, He it is who absolves, although men are appointed by Him the external ministers of the Sacraments—so has He placed over His Church, which He governs by His invisible Spirit, a man to be His vicar and the minister of His power. A visible Church requires a visible head; therefore the Saviour appointed Peter head and pastor of all the faithful, when He committed to his care the feeding of all His sheep, in such ample terms that He willed the very same power of ruling and governing the entire Church to descend to Peter's successors.
Unity In Spirit, Hope And Faith
Moreover, the Apostle, writing to the Corinthians, tells them that there is but one and the same Spirit who imparts grace to the faithful, as the soul communicates life to the members of the body. Exhorting the Ephesians to preserve this unity, he says: Be careful to keep the unity of the Spirit in the bond of peace; one body and one Spirit. As the human body consists of many members, animated by one soul, which gives sight to the eyes, hearing to the ears, and to the other senses the power of discharging their respective functions; so the mystical body of Christ, which is the Church, is composed of many faithful. The hope, to which we are called, is also one, as the Apostle tells us in the same place; for we all hope for the same consummation, eternal and happy life. Finally, the faith which all are bound to believe and to profess is one: Let there be no schisms amongst you, says the Apostle. And Baptism, which is the seal of our Christian faith, is also one.