For this reason the Church is so often called in Holy Writings a body, and even the body of Christ – ‘Now you are the body of Christ’ (1Cor 12:27) – and precisely because it is a body is the Church visible: and because it is the body of Christ is it living and energizing, because by the infusion of His power Christ guards and sustains it, just as the vine gives nourishment and renders fruitful the branches united to it. And as in animals the vital principle is unseen and invisible, and is evidenced and manifested by the movements and action of the members, so the principle of supernatural life in the Church is clearly shown in that which is done by it. From this it follows that those who arbitrarily conjure up and picture to themselves a hidden and invisible Church are in grievous and pernicious error: as also are those who regard the Church as a human institution which claims a certain obedience in discipline and external duties, but which is without the perennial communication of the gifts of divine grace, and without all that which testifies by constant and undoubted signs to the existence of that life which is drawn from God. (Leo XIII. Encyclical Satis cognitum, no. 3, June 29, 1896)

And since the glory of God is Love, the heavenly Jerusalem is the icon of the Church, utterly holy and glorious, without spot or wrinkle (cf. Eph 5:27), permeated at her heart and in every part of her by the presence of the God who is Love. She is called a ‘bride’, ‘the bride of the Lamb’ (Rev 20:9), […] The City and Bride is the locus of God’s full communion with humanity; She has no need of a temple or of any external source of light, because the indwelling presence of God and of the Lamb illuminates her from within. (Benedict XVI. Holy Mass for the inauguration of the Fifth General Conference of the Bishops of Latin America and the Caribbean, May 13, 2007)

The Church lives not for herself, but for Christ. She wants to be the ‘star’, the point of reference which helps people find the path which leads to him. The theology of the Fathers loved to speak of the Church as mysterium lunae, in order to emphasize that, like the moon, she shines not with her own light, but reflects Christ, who is her Sun. And I gladly recall that this is how the Second Vatican Council’s Dogmatic Constitution on the Church begins: ‘Christ is the light of the nations, lumen gentium!’ And the Council Fathers went on to express their burning desire to ‘enlighten all people with the light of Christ reflected on the face of the Church’ (no. 1). (John Paul II, Homily during the Closing of the Holy Door on the Solemnity of the Epiphany, January 6, 2001)

As the nerves extend from the head to all parts of the human body and give them power to feel and to move, in like manner our Savior communicates strength and power to His Church so that the things of God are understood more clearly and are more eagerly desired by the faithful. From Him streams into the body of the Church all the light with which those who believe are divinely illumined, and all the grace by which they are made holy as He is holy. Christ enlightens His whole Church […] It is He who imparts the light of faith to believers; it is He who enriches pastors and teachers and above all His Vicar on earth with the supernatural gifts of knowledge, understanding and wisdom, so that they may loyally preserve the treasury of faith, defend it vigorously, and explain it and confirm it with reverence and devotion. Finally, it is He who, though unseen, presides at the Councils of the Church and guides them. (Pius XII. Encyclical Mystici Corporis Christi, June 29, 1943)

This appellation of the Body of Christ is not to be explained solely by the fact that Christ must be called the Head of His Mystical Body, but also by the fact that He so sustains the Church, and so in a certain sense lives in the Church, that she is, as it were, another Christ. […] Nevertheless this most noble title of the Church must not be so understood as if that ineffable bond by which the Son of God assumed a definite human nature belongs to the universal Church; but it consists in this, that our Savior shares prerogatives peculiarly His own with the Church in such a way that she may portray, in her whole life, both exterior and interior, a most faithful image of Christ. (Pius XII. Encyclical Mystici Corporis Christi, June 29, 1943)

Not from her own light does the Church gleam, but from the light of Christ. From the Sun of Justice has her brilliance been obtained, so that it is said: ‘It is now no longer I that live, but Christ lives in me’ (cf. Gal 2:20) (St. Ambrose. Hexameron, Six days of creation: Four, Ch.8, 32)

The Lord Jesus, the only Savior, did not only establish a simple community of disciples, but constituted the Church as a salvific mystery: he himself is in the Church and the Church is in him (cf. Jn 15:1ff.; Gal 3:28; Eph 4:15-16; Acts 9:5).  Therefore, the fullness of Christ’s salvific mystery belongs also to the Church, inseparably united to her Lord. Indeed, Jesus Christ continues his presence and his work of salvation in the Church and by means of the Church (cf. Col 1:24-27), which is his body (cf. 1Cor 12:12-13, 27; Col 1:18). And thus, just as the head and members of a living body, though not identical, are inseparable, so too Christ and the Church can neither be confused nor separated, and constitute a single ‘whole Christ’. (Congregation for the Doctrine of the Faith, Declaration Dominus Jesus, no. 16, August 6, 2000)

Christ loved the church and handed himself over for her to sanctify her, cleansing her by the bath of water with the word, that he might present to himself the church in splendor, without spot or wrinkle or any such thing, that she might be holy and without blemish. (Eph 5:25-27)

To use the words of the fathers of Trent, it is certain that the Church ‘was instructed by Jesus Christ and His Apostles and that all truth was daily taught it by the inspiration of the Holy Spirit’ (sess. 13, prooemium). Therefore, it is obviously absurd and injurious to propose a certain ‘restoration and regeneration’ for Her as though necessary for Her safety and growth, as if She could be considered subject to defect or obscuration or other misfortune. Indeed these authors of novelties consider that a ‘foundation may be laid of a new human institution,’ and what Cyprian detested may come to pass, that what was a divine thing ‘may become a human church.’ (Gregory XVI, Encyclical Mirari vos, August 15, 1832)

But if our Savior, by His death, became, in the full and complete sense of the word, the Head of the Church, it was likewise through His blood that the Church was enriched with the fullest communication of the Holy Spirit, through which, from the time when the Son of Man was lifted up and glorified on the Cross by His sufferings, She is divinely illumined. […] so at the hour of His precious death He willed that His Church should be enriched with the abundant gifts of the Paraclete in order that in dispensing the divine fruits of the Redemption She might be, for the Incarnate Word, a powerful instrument that would never fail. For both the juridical mission of the Church, and the power to teach, govern and administer the Sacraments, derive their supernatural efficacy and force for the building up of the Body of Christ from the fact that Jesus Christ, hanging on the Cross, opened up to His Church the fountain of those divine gifts, which prevent her from ever teaching false doctrine and enable her to rule them for the salvation of their souls through divinely enlightened pastors and to bestow on them an abundance of heavenly graces. (Pius XII. Encyclical Mystici Corporis Christi, June 29, 1943)

Sanctity constitutes the profound identity of the Church as the Body of Christ, vivified and participant of his Spirit. Sanctity gives spiritual health to the Body. Sanctity also determines its spiritual beauty: that beauty that surpasses all the beauty of nature and art; a supernatural beauty, in which the beauty of God himself is reflected, in a more essential and direct way than all of the beauty of creation, precisely because it is the Corpus Christi. (John Paul II, General Audience, November 28, 1990)

There are also conceptions which deliberately emphasize […] of a Church which is not concerned about herself, but which is totally concerned with bearing witness to and serving the kingdom. It is a ‘Church for others just as Christ is the ‘man for others.’ The Church’s task is described as though it had to proceed in two directions: on the one hand promoting such ‘values of the kingdom’ as peace, justice, freedom, brotherhood, etc., while on the other hand fostering dialogue between peoples, cultures and religions, so that through a mutual enrichment they might help the world to be renewed and to journey ever closer toward the kingdom. […] Furthermore, the kingdom, as they understand it, ends up either leaving very little room for the Church or undervaluing the Church in reaction to a presumed ‘ecclesiocentrism’ of the past. […] One need not fear falling thereby into a form of ‘ecclesiocentrism.’ Pope Paul VI, who affirmed the existence of ‘a profound link between Christ, the Church and evangelization,’ also said that the Church ‘is not an end unto herself, but rather is fervently concerned to be completely of Christ, in Christ and for Christ, as well as completely of men, among men and for men.’ (John Paul II. Encyclical Redemptoris missio, no.17, December 7, 1990)

The Church wishes, in the end, to contemplate itself; or rather, to seek herself in the mind of Christ, her divine Founder; this certainly is the same as rendering honor to the wisdom and charity of her Founder and — while professing faith and fidelity with continual observance — making herself even more fit to carry out the mission of salvation, for which she was instituted. No one, however, should think that the Church, in so doing, closes in on herself to auto-delight in herself and forgets either Christ, from whom she receives all and to whom she owes all, or humanity, for whose service she was born. The Church is in between Christ and the human community, not turned in on herself, nor as an opaque veil that obscures vision, not oriented towards itself; but rather, on the contrary, constantly seeking to be entirely of Christ, in Christ, for Christ, to be entirely of men, among men, for men, a truly humble and excellent intermediary between the Divine Savior and humanity, instituted for the defending and spreading of the truth and the grace of supernatural life. (Paul VI. Inauguration of the Third Session of the Second Vatican Council, September 14, 1964)

In the formation of their consciences, the Christian faithful ought carefully to attend to the sacred and certain doctrine of the Church. For the Church is, by the will of Christ, the teacher of the truth. It is her duty to give utterance to, and authoritatively to teach, that truth which is Christ Himself, and also to declare and confirm by her authority those principles of the moral order which have their origins in human nature itself. […] The disciple is bound by a grave obligation toward Christ, his Master, ever more fully to understand the truth received from Him, faithfully to proclaim it, and vigorously to defend it. (Vatican Council II, Declaration Dignitaries humane, no. 14, December 7, 1965)

For Bishops are preachers of the faith, who lead new disciples to Christ, and they are authentic teachers, that is, teachers endowed with the authority of Christ, who preach to the people committed to them the faith they must believe and put into practice, and by the light of the Holy Spirit illustrate that faith. They bring forth from the treasury of Revelation new things and old (cf. Mt 13:52), making it bear fruit and vigilantly warding off any errors that threaten their flock (2 Tim 4:1-4). (Vatican Council II, Dogmatic Constitution Lumen gentium, November 21, 1964)

But by divine institution it is the exclusive task of these pastors alone, the successors of Peter and the other Apostles, to teach the faithful authentically, that is with the authority of Christ shared in different ways; […] The People of God has particular need of the intervention and assistance of the Magisterium when internal disagreements arise and spread concerning a doctrine that must be believed or held, lest it lose the communion of the one faith in the one Body of the Lord (cf. Eph 4:4, 5). (Congregation for the Doctrine of the Faith, Declaration Mysterium ecclesiae, no. 2, June 24, 1973)

The See of St. Peter always remains unimpaired by any error, according to the divine promise of our Lord the Savior made to the chief of His disciples: “I have prayed for thee, that thy faith fail not: and thou, being once converted, confirm thy brethren” (Lk 22,32). So, this gift of truth and a never failing faith was divinely conferred upon Peter and his successors in this chair, that they might administer their high duty for the salvation of all; that the entire flock of Christ, turned away by them from the poisonous food of error, might be nourished on the sustenance of heavenly doctrine, that with the occasion of schism removed the whole Church might be saved as one, and relying on her foundation might stay firm against the gates of hell. (Denzinger-Hünermann, 3070-3071, Vatican Council I. Session IV, On the Church of Christ, Ch. 4, July 18, 1870)

But some are more easily deceived by the outward appearance of good when there is question of fostering unity among all Christians. Is it not right, it is often repeated, indeed, even consonant with duty, that all who invoke the name of Christ should abstain from mutual reproaches and at long last be united in mutual charity? […] These things and others that class of men who are known as pan-Christians continually repeat and amplify; and these men, so far from being quite few and scattered, have increased to the dimensions of an entire class, and have grouped themselves into widely spread societies, most of which are directed by non-Catholics, although they are imbued with varying doctrines concerning the things of faith. […] These pan-Christians who turn their minds to uniting the churches seem, indeed, to pursue the noblest of ideas in promoting charity among all Christians: nevertheless how does it happen that this charity tends to injure faith? Everyone knows that John himself, the Apostle of love, who seems to reveal in his Gospel the secrets of the Sacred Heart of Jesus, and who never ceased to impress on the memories of his followers the new commandment ‘Love one another,’ altogether forbade any intercourse with those who professed a mutilated and corrupt version of Christ’s teaching: ‘If any man come to you and bring not this doctrine, receive him not into the house nor say to him: God speed you.’ [18] For which reason, since charity is based on a complete and sincere faith, the disciples of Christ must be united principally by the bond of one faith. (Pius XI. Encyclical Mortalium animos, no. 3-4, 9, January 6, 1928)

For the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it. To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it. […] Furthermore, in this one Church of Christ no man can be or remain who does not accept, recognize and obey the authority and supremacy of Peter and his legitimate successors. Did not the ancestors of those who are now entangled in the errors of Photius and the reformers, obey the Bishop of Rome, the chief shepherd of souls? Alas their children left the home of their fathers, but it did not fall to the ground and perish forever, for it was supported by God. […] Let, therefore, the separated children draw nigh to the Apostolic See, set up in the City which Peter and Paul, the Princes of the Apostles, consecrated by their blood; to that See, We repeat, which is ‘the root and womb whence the Church of God springs,’(S. Cyprian Ep. 48 ad Cornelium, 3) not with the intention and the hope that ‘the Church of the living God, the pillar and ground of the truth’ (1 Tim 3:15) will cast aside the integrity of the faith and tolerate their errors, but, on the contrary, that they themselves submit to its teaching and government.  (Pius XI, Encyclical Mortalium animos, no. 10, 11, 12, January 6, 1928)

The Church, founded on these principles and mindful of her office, has done nothing with greater zeal and endeavour than she has displayed in guarding the integrity of the faith. Hence she regarded as rebels and expelled from the ranks of her children all who held beliefs on any point of doctrine different from her own. […] The practice of the Church has always been the same, as is shown by the unanimous teaching of the Fathers, who were wont to hold as outside Catholic communion, and alien to the Church, whoever would recede in the least degree from any point of doctrine proposed by her authoritative Magisterium. […] The need of this divinely instituted means for the preservation of unity, about which we speak is urged by St. Paul in his epistle to the Ephesians. In this he first admonishes them to preserve with every care concord of minds: “Solicitous to keep the unity of the Spirit in the bond of peace’ (Eph. 4:3). And as souls cannot be perfectly united in charity unless minds agree in faith, he wishes all to hold the same faith: ‘One Lord, one faith.’ (Leo XIII. Encyclical Satis cognitum, no. 9, June 29, 1896)

Hence the teaching of Cyprian, that heresy and schism arise and are begotten from the fact that due obedience is refused to the supreme authority. ‘Heresies and schisms have no other origin than that obedience is refused to the priest of God, and that men lose sight of the fact that [in the Church] there is only one priest and only one judge, in the place of Christ.’ (Leo XIII. Encyclical Satis cognitum, no. 15, June 29, 1896)

We believe it our duty and office to cut and uproot the noxious weeds we perceive growing, so that they do not take root and spread damage in the field of the Lord. Certainly, ever since the origins of the early Church, due to the necessity that the faith of the elect be proven like gold in the fire, the Apostle, vessel of election, wished to warn the faithful that if someone arise, of those who alter and amend the Gospel of Christ, spreading false doctrines and betraying the depository of the faith, even if it was an angel who evangelized something other than he had, it would be necessary to anathemize him. (Pius IX, Apostolic Letter Ad Apostolicae Sedis, August 22, 1851)

According to Wisdom 11:11 – ‘By what things a man sinneth, by the same also he should be punished.’ Now a schismatic, as shown above (article 1), commits a twofold sin: first by separating himself from communion with the members of the Church, and in this respect the fitting punishment for schismatics is that they be excommunicated. Secondly, they refuse submission to the head of the Church, wherefore, since they are unwilling to be controlled by the Church’s spiritual power, it is just that they should be compelled by the secular power. (Saint Thomas Aquinas, Summa Theologica II-II, q. 39, a.4)