According to Acts, the unity of believers was seen in the fact that “they devoted themselves to the Apostles’ teaching and fellowship, to the breaking of the bread and the prayers” (2:42). The unity of believers was thus nourished by the teaching of the Apostles (the proclamation of God’s word), to which they responded with unanimous faith, by fraternal communion (the service of charity), by the breaking of the bread (the Eucharist and the sacraments), and by prayer, both personal and communal. It was on these four pillars that communion and witness were based within the first community of believers. (Benedict XVI. Apostolic exhortation Ecclesia in Medio Oriente, no. 5, September 14, 2012)

The Church is a ‘communion of saints’: this expression refers first to the ‘holy things’ (sancta), above all the Eucharist, by which ‘the unity of believers, who form one body in Christ, is both represented and brought about’ (LG 3). (Catechism of the Catholic Church, no. 960)

In this sacrament, as in the others, that which is a sacrament is a sign of the reality of the sacrament. Now there is a twofold reality of this sacrament, as stated above (q. 73, a. 6): one which is signified and contained, namely, Christ Himself; while the other is signified but not contained, namely, Christ’s mystical body, which is the fellowship of the saints. Therefore, whoever receives this sacrament, expresses thereby that he is made one with Christ, and incorporated in His members. (Saint Thomas Aquinas. Summa Theologica III, q. 80, a. 4)

This sacrament has a threefold significance. One with regard to the past, inasmuch as it is commemorative of our Lord’s Passion, which was a true sacrifice, as stated above (q. 48, a. 3), and in this respect it is called a ‘Sacrifice.’ With regard to the present it has another meaning, namely, that of Ecclesiastical unity, in which men are aggregated through this Sacrament; and in this respect it is called ‘Communion’ or ‘Synaxis’. For Damascene says (De Fide Orth. IV) that ‘it is called Communion because we communicate with Christ through it, both because we partake of His flesh and Godhead, and because we communicate with and are united to one another through it.’ (Saint Thomas Aquinas. Summa Theologica III, q. 73, a. 4)

Receive, therefore, and eat the body of Christ; now that you too have become members of Christ in the body of Christ; receive and drink the blood of Christ. So as not to be separated, eat what unites you; in order not to seem cheap in your own estimation, drink the price paid for you. […] So then, if you have life in him, you will be with him in one flesh. This sacrament, after all, does not give you the body of Christ so as then to divide you in it. […] You, then, begin to receive what you have already begun to be, provided that you do not receive it unworthily, eating and drinking your own condemnation. […] And you receive it worthily, if you keep far from the yeast of bad doctrines, so that you be ‘unleavened loaves of sincerity and truth’ (1Cor 5:8). (Saint Augustine. Sermon 228 B, About the Sacraments, on Easter Day, no. 3–5)

He wished, furthermore, that this be a pledge of our future glory and of everlasting happiness, and thus be a symbol of that one ‘body’ of which He Himself is the ‘head’ (1Cor 11:23 Eph 5:23), and to which He wished us to be united, as members, by the closest bond of faith, hope, and charity, that we might ‘all speak the same thing and there might be no schisms among us’ (cf. 1Cor 1:10). (Denzinger-Hünermann 1638. Julius III, Council of Trent, Session XIII, October 11, 1551)

But although there are two elements, as bread and wine, of which the entire Sacrament of the Eucharist is constituted, yet guided by the authority of the Church, we confess that this is not many Sacraments, but only one. […] Moreover, by virtue of the Sacrament, one mystical body is effected; hence, that the Sacrament itself may correspond to the thing which it effects, it must be one. (Catechism of Trent, no. 2300)

The promotion of unity belongs to the innermost nature of the Church, for she is, ‘thanks to her relationship with Christ, a sacramental sign and an instrument of intimate union with God, and of the unity of the whole human race’ (cf. Sir. 15:14). Thus she shows the world that an authentic union, social and external, results from a union of minds and hearts, namely from that faith and charity by which her own unity is unbreakably rooted in the Holy Spirit. For the force which the Church can inject into the modern society of man consists in that faith and charity put into vital practice, not in any external dominion exercised by merely human means. (Vatican Council II. Pastoral constitution Gaudium et spes, no. 42, December 7, 1965)

What are these bonds of unity? Above all, charity ‘binds everything together in perfect harmony’ (Col 3:14). But the unity of the pilgrim Church is also assured by visible bonds of communion: – profession of one faith received from the Apostles; common celebration of divine worship, especially of the sacraments; apostolic succession through the sacrament of Holy Orders, maintaining the fraternal concord of God’s family. (Catechism of the Catholic Church, no. 815)

But mark this well: unless the faithful remain bound together by the same ties of virtue, worship and sacrament, and all hold fast to the same belief, they cannot be perfectly united with the Divine Redeemer, the universal Head, so as to form with Him one visible and living body. “A whole faith,” says St. Leo, “a true faith, is a mighty bulwark. No one can add anything to it, no one can take anything away from it; for unless it is one, it is no faith at all.” To preserve this unity of faith, all teachers of divine truths—all bishops, that is—must necessarily speak with one mind and one voice. (John XXIII. Encyclical Aeterna Dei sapientia, November 11, 1961)

Love for the truth is the deepest dimension of any authentic quest for full communion between Christians. […] Full communion of course will have to come about through the acceptance of the whole truth into which the Holy Spirit guides Christ’s disciples. (John Paul II. Encyclical Ut unum sint, no. 36, May 25, 1995)

Unity in the truth: this is the mission Christ entrusted to His Church, for which she works actively, invoking it first and foremost of Him who can do all things and who, when His passion and resurrection were imminent, first prayed to the Father that all believers might be “one” (Jn 17:21). […] it is made clear that this mysterious, visible union cannot be pursued without an identity of faith, a sharing in the sacramental life, the resulting consistency in moral life and continuous, fervent personal and communal prayer. (John Paul II. Address during the official release of the new Catechism of the Catholic Church, December7, 1992)

The only successful method will be that which bases harmony and agreement among Christ’s faithful ones upon all the truths, and the whole of the truths, which God has revealed. (Pius XII. Enyclical Orientalis Ecclesiae, no. 16, April 9, 1944)

If, as We desire with all Our heart, the highest possible peak of well being for society and its members is to be attained through fraternity or, as it is also called, universal solidarity, all minds must be united in the knowledge of Truth, all wills united in morality, and all hearts in the love of God and His Son Jesus Christ. (Pius X. Encyclical Notre charge apostolique, August 23, 1910)

The True Union between Christians is that which Jesus Christ, the Author of the Church, instituted and desired, and which consists in a unity of faith and unity of government. (Leo XIII. Encyclical Praeclara gratulationis, no. 8, June 20, 1894)

It is the Holy Spirit, the principle of unity, which establishes the Church as a communion. He is the principle of the unity of the faithful in the teaching of the Apostles, in the breaking of the bread and in prayer. […] The communion of the baptized in the teaching of the Apostles and in the breaking of the eucharistic bread is visibly manifested in the bonds of the profession of the faith in its entirety, of the celebration of all of the sacraments instituted by Christ, and of the governance of the College of Bishops united with its head, the Roman Pontiff. (Benedict XVI. Apostolic constitution Anglicanorum coetibus, November 4, 2009)

The preacher of the Gospel […] never betrays or hides truth out of a desire to please men. (Paul VI. Apostolic exhortation Evangelii nuntiandi, no. 78, December 8, 1975)

But when devotional exercises, and pious practices in general, not strictly connected with the sacred liturgy, confine themselves to merely human acts, with the express purpose of directing these latter to the Father in heaven, of rousing people to repentance and holy fear of God, of weaning them from the seductions of the world and its vice, and leading them back to the difficult path of perfection, then certainly such practices are not only highly praiseworthy but absolutely indispensable, because they expose the dangers threatening the spiritual life; because they promote the acquisition of virtue; and because they increase the fervor and generosity with which we are bound to dedicate all that we are and all that we have to the service of Jesus Christ. Genuine and real piety, which the Angelic Doctor calls “devotion,“ and which is the principal act of the virtue of religion — that act which correctly relates and fitly directs men to God; and by which they freely and spontaneously give themselves to the worship of God in its fullest sense – piety of this authentic sort needs meditation on the supernatural realities and spiritual exercises, if it is to be nurtured, stimulated and sustained, and if it is to prompt us to lead a more perfect life. (Pius XII. Encyclical Mediator Dei, no. 45-46, December 20, 1947)

This explains why the Church in a brief and significant phrase calls the various acts of mortification, especially those practiced during the season of Lent, “the Christian army’s defenses” (Roman Missal, Ash Wednesday). They represent, in fact, the personal effort and activity of members who desire, as grace urges and aids them, to join forces with their Captain – “that we may discover . . . in our Captain,” to borrow St. Augustine’s words, “the fountain of grace itself” (De praedestinatione sanctorum, 31). But observe that these members are alive, endowed and equipped with an intelligence and will of their own. It follows that they are strictly required to put their own lips to the fountain, imbibe and absorb for themselves the life-giving water, and rid themselves personally of anything that might hinder its nutritive effect in their souls. Emphatically, therefore, the work of redemption, which in itself is independent of our will, requires a serious interior effort on our part if we are to achieve eternal salvation. (Pius XII. Encyclical Mediator Dei, no. 44, November 20, 1947)

For from Adam unto Moses the human race lived of the body, that is, according to the flesh: which is called the outward and the old man, and to which the Old Testament was given, that it might prefigure the spiritual things to come by operations, albeit religious, yet carnal. Through this entire season, when men lived according to the body, “death reigned,” as the Apostle says, “even over those that had not sinned.” Now it reigned “after the similitude of Adam’s transgression” (Rom 5:14), as the same Apostle says; for it must be taken of the period up to Moses, up to which time the works of the law, that is, those sacraments of carnal observance, held even those bound, for the sake of a certain mystery, who were subject to the One God. But from the coming of the Lord, from whom there was a transition from the circumcision of the flesh to the circumcision of the heart, the call was made, that man should live according to the soul, that is, according to the inner man, who is also called the “new man” (Col 3:10) by reason of the new birth and the renewing of spiritual conversation. (Saint Augustine of Hippo. Expositions of the Psalms, Psalm 6, no. 2)

I know that there are many who in words have renounced this world, and yet desire to be burdened with all the weight of worldly things, and rejoice in such burdens. Nor is it surprising that among so many multitudes you should find some by condemning whose life you may deceive the unwary and seduce them from Catholic safety; for in your small numbers you are at a loss when called on to show even one out of those whom you call the elect who keeps the precepts, which in your indefensible superstition you profess. (Saint Augustine of Hippo. Of the morals of the Catholic Church, Ch. 34, no. 75)

As Isidore says (Etym. X), “according to Cicero, a man is said to be religious from ‘religio,’ because he often ponders over, and, as it were, reads again [relegit], the things which pertain to the worship of God,” so that religion would seem to take its name from reading over those things which belong to Divine worship because we ought frequently to ponder over such things in our hearts, according to Prov. 3:6, “In all thy ways think on Him.” According to Augustine (De Civ. Dei X, 3) it may also take its name from the fact that “we ought to seek God again, whom we had lost by our neglect” [St. Augustine plays on the words ‘reeligere’ (to choose over again) and ‘negligere’ (to neglect or despise)]. Or again, religion may be derived from “religare” [to bind together], wherefore Augustine says (De Vera Relig. 55): “May religion bind us to the one Almighty God.” However, whether religion take its name from frequent reading, or from a repeated choice of what has been lost through negligence, or from being a bond, it denotes properly a relation to God. For it is He to Whom we ought to be bound as to our unfailing principle; to Whom also our choice should be resolutely directed as to our last end; and Whom we lose when we neglect Him by sin, and should recover by believing in Him and confessing our faith. […] “It is justice whereby men both will end do just actions.” Now it is evident that to do what pertains to the worship or service of God, belongs properly to religion, as stated above (q. 81). Wherefore it belongs to that virtue to have the will ready to do such things, and this is to be devout. Hence it is evident that devotion is an act of religion. (Saint Thomas Aquinas. Summa Theologica II–II, q. 81 a. 1; q. 82, a. 2)

Popular devotions of the Christian people are to be highly commended, provided they accord with the laws and norms of the Church, above all when they are ordered by the Apostolic See. Devotions proper to individual Churches also have a special dignity if they are undertaken by mandate of the bishops according to customs or books lawfully approved. (Vatican Council II. Constitution Sacrosanctum Concilium, no. 13, December 4, 1963)

The Holy Gospel narrates that when Jesus was crucified “here was darkness over the whole earth” (Mt 27:45); a terrifying symbol of what happened and what still happens spiritually wherever incredulity, blind and proud of itself, has succeeded in excluding Christ from modern life, especially from public life, and has undermined faith in God as well as faith in Christ. The consequence is that the moral values by which in other times public and private conduct was gauged have fallen into disuse; […] Many perhaps, while abandoning the teaching of Christ, were not fully conscious of being led astray by a mirage of glittering phrases, which proclaimed such estrangement as an escape from the slavery in which they were before held; nor did they then foresee the bitter consequences of bartering the truth that sets free, for error which enslaves. They did not realize that, in renouncing the infinitely wise and paternal laws of God, and the unifying and elevating doctrines of Christ’s love, they were resigning themselves to the whim of a poor, fickle human wisdom; […] They did not perceive the inability of all human effort to replace the law of Christ by anything equal to it; “they became vain in their thoughts” (Rom 1:21). With the weakening of faith in God and in Jesus Christ, and the darkening in men’s minds of the light of moral principles, there disappeared the indispensable foundation of the stability and quiet of that internal and external, private and public order, which alone can support and safeguard the prosperity of States. (Pius XII. Encyclical Summi pontificatus, no. 30.31–32, October 20, 1939)

This is the religion which possesses the universal way for delivering the soul; for except by this way, none can be delivered. This is a kind of royal way, which alone leads to a kingdom which does not totter like all temporal dignities, but stands firm on eternal foundations. […] that no system of doctrine which furnishes the universal way for delivering the soul has as yet been received […] For what else is the universal way of the soul’s deliverance than that by which all souls universally are delivered, and without which, therefore, no soul is delivered? […] What is this universal way of which he acknowledges his ignorance, if not a way which does not belong to one nation as its special property, but is common to all, and divinely bestowed? Porphyry, a man of no mediocre abilities, does not question that such a way exists; for he believes that Divine Providence could not have left men destitute of this universal way of delivering the soul. […] This, then, is the universal way of the soul’s deliverance […] the grace of God […] This way purifies the whole man, and prepares the mortal in all his parts for immortality. […] Except by this way, which has been present among men both during the period of the promises and of the proclamation of their fulfillment, no man has been delivered, no man is delivered, no man shall be delivered. (Saint Augustine of Hippo. The City of God, Book X, Ch. 32)

We must not ignore the fact that many, even generous Christians […] are frequently tempted to reduce her mission to the dimensions of a simply temporal project. They would reduce her aims to a man-centered goal; the salvation of which she is the messenger would be reduced to material well-being. Her activity, forgetful of all spiritual and religious preoccupation, would become initiatives of the political or social order. But if this were so, the Church would lose her fundamental meaning. Her message of liberation would no longer have any originality and would easily be open to monopolization and manipulation by ideological systems and political parties. She would have no more authority to proclaim freedom as in the name of God. This is why we have wished to emphasize, in the same address at the opening of the Synod, “[…] the kingdom of God, before anything else, in its fully theological meaning….” […] the Church is certainly not willing to restrict her mission only to the religious field and dissociate herself from man’s temporal problems. Nevertheless she reaffirms the primacy of her spiritual vocation and refuses to replace the proclamation of the kingdom by the proclamation of forms of human liberation- she even states that her contribution to liberation is incomplete if she neglects to proclaim salvation in Jesus Christ. (Paul VI. Apostolic exhortation Evangelii nuntiandi, no. 32–34, December 8, 1975)

As the kernel and center of His Good News, Christ proclaims salvation, this great gift of God which is liberation from everything that oppresses man but which is above all liberation from sin and the Evil One, in the joy of knowing God and being known by Him, of seeing Him, and of being given over to Him. (Paul VI. Apostolic exhortation Evangelii nuntiandi, no. 9, December 8, 1975)

This truth which comes from God has its centre in Jesus Christ, the Saviour of the world. From him, who is “the way, and the truth, and the life“ (Jn 14:6), the Church receives all that she has to offer to mankind. Through the mystery of the Incarnate Word and Redeemer of the world, she possesses the truth regarding the Father and his love for us, and also the truth concerning man and his freedom. Through his Cross and Resurrection, Christ has brought about our Redemption, which is liberation in the strongest sense of the word, since it has freed us from the most radical evil, namely sin and the power of death. (Congregation for the Doctrine of the Faith. Instruction Libertatis conscientia, no. 3, March 22, 1896)

It is only when one begins with the task of evangelization understood in its entirety that the authentic requirements of human progress and liberation are appreciated. This liberation has as its indispensable pillars: ‘the truth about Jesus the Savior‘; ‘the truth about the Church‘; and ‘the truth about man and his dignity’. It is in light of the Beatitudes, and especially the Beatitude of the poor of heart, that the Church, which wants to be the Church of the poor throughout the world, intends to come to the aid of the noble struggle for truth and justice. She addresses each person, and for that reason, every person. (Congregation for the Doctrine of the Faith. Instruction on certain aspects of the “Theology of Liberation”, XI, no. 5, August 6, 1984)

And as Pastors you have the vivid awareness that your principal duty is to be Teachers of the Truth. Not a human and rational truth, but the Truth that comes from God, the Truth that brings with it the principle of the authentic liberation of man: ‘you will know the truth, and the truth will make you free’ (Jn 8:32); that Truth which is the only one that offers a solid basis for an adequate “praxis”. (John Paul II. Address at the Third General Conference of the Latin American Episcopate, January 28, 1979)

Thereupon there came into being and spread far and wide throughout the world that doctrine of rationalism or naturalism, — utterly opposed to the Christian – religion, since this is of supernatural origin, — which spares no effort to bring it about that Christ, who alone is our Lord and Savior, is shut out from the minds of people and the moral life of nations. Thus they would establish what they call the rule of simple reason or nature. The abandonment and rejection of the Christian religion, and the denial of God and his Christ, has plunged the minds of many into the abyss of pantheism, materialism and atheism, and the consequence is that they strive to destroy rational nature itself, to deny any criterion of what is right and just, and to overthrow the very foundations of human society. With this impiety spreading in every direction, it has come about, alas, that many even among the children of the Catholic Church have strayed from the path of genuine piety, and as the truth was gradually diluted in them, their Catholic sensibility was weakened. (Vatican Council I. Dogmatic constitution Dei Filius, Session 3, no. 7–8, April 24, 1870)