One body and one Spirit, as you were also called to the one hope of your call; one Lord, one faith, one baptism; one God and Father of all, who is over all and through all and in all. (Eph 4:4-6)

No, I mean that what they sacrifice, (they sacrifice) to demons, not to God, and I do not want you to become participants with demons. You cannot drink the cup of the Lord and also the cup of demons. You cannot partake of the table of the Lord and of the table of demons. Or are we provoking the Lord to jealous anger? Are we stronger than he? (1Cor 10:20-22)

Do not be yoked with those who are different, with unbelievers. For what partnership do righteousness and lawlessness have? Or what fellowship does light have with darkness? What accord has Christ with Beliar? Or what has a believer in common with an unbeliever? What agreement has the temple of God with idols? For we are the temple of the living God; as God said: ‘I will live with them and move among them, and I will be their God and they shall be my people. Therefore, come forth from them and be separate,’ says the Lord, ‘and touch nothing unclean; then I will receive you and I will be a father to you, and you shall be sons and daughters to me, says the Lord Almighty’. Since we have these promises, beloved, let us cleanse ourselves from every defilement of flesh and spirit, making holiness perfect in the fear of God. (2Cor 6:14-18. 7:1)

In the course of time, namely from the beginnings of the human race until the coming and preaching of Jesus Christ, He Himself taught man the duties which a rational creature owes to its Creator […] From which it follows that there can be no true religion other than that which is founded on the revealed word of God: which revelation, begun from the beginning and continued under the Old Law, Christ Jesus Himself under the New Law perfected. Now, if God has spoken (and it is historically certain that He has truly spoken), all must see that it is man’s duty to believe absolutely God’s revelation and to obey implicitly His commands; that we might rightly do both, for the glory of God and our own salvation, the Only-begotten Son of God founded His Church on earth. (Pius XI. Encyclical Mortalium Animos, January 6, 1928)

The Lord Jesus, the only Saviour, 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 15:1 ff.; Gal 3:28; Eph 4:15-16; Acts 9:5). […] The Catholic faithful are required to profess that there is an historical continuity — rooted in the apostolic succession — between the Church founded by Christ and the Catholic Church: ‘This is the single Church of Christ… which our Saviour, after his resurrection, entrusted to Peter’s pastoral care (cf. Jn 21:17), commissioning him and the other Apostles to extend and rule her (cf. Mt 28:18ff.), erected for all ages as ‘the pillar and mainstay of the truth’ (1 Tim 3:15). (Congregation for the Doctrine of the Faith, Declaration Dominus Iesus, on the unicity and salvific universality of Jesus Christ and the Church, IV, 16)

Moreover the blessed Apostle Paul teaches the same thing, and sets forth the sacrament of unity, saying, ‘There is one body and one spirit, one hope of your calling, one Lord, one faith, one baptism, one God?’ And this unity we ought firmly to hold and assert, especially those of us that are bishops who preside in the Church, that we may also prove the episcopate itself to be one and undivided. Let no one deceive the brotherhood by a falsehood: let no one corrupt the truth of the faith by perfidious prevarication. […] Thus also the Church, Shone over with the light of the Lord, Sheds forth her rays over the whole world, yet it is one light which is everywhere diffused, nor is the unity of the body separated. Her fruitful abundance spreads her branches over the whole world. She broadly expands her rivers, liberally flowing, yet her head is one, her source one; and she is one mother, plentiful in the results of fruitfulness: from her womb we are born, by her milk we are nourished, by her spirit we are animated. (Saint Cyprian of Carthage, De unitate Ecclesiae, no. 4-5)

The spouse of Christ cannot be adulterous; she is uncorrupted and pure. She knows one home; she guards with chaste modesty the sanctity of one couch. She keeps us for God. She appoints the sons whom she has born for the kingdom. Whoever is separated from the Church and is joined to an adulteress, is separated from the promises of the Church; nor can he who forsakes the Church of Christ attain to the rewards of Christ. He is a stranger; he is profane; he is an enemy. He can no longer have God for his Father, who has not the Church for his mother. If any one could escape who was outside the ark of Noah, then he also may escape who shall be outside of the Church. The Lord warns, saying, ‘He who is not with me is against me, and he who gathereth not with me scattereth.’ He who breaks the peace and the concord of Christ, does so in opposition to Christ. (Saint Cyprian of Carthage. De unitate Ecclesiae, no. 6)

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? Who would dare to say that he loved Christ, unless he worked with all his might to carry out the desires of Him, Who asked His Father that His disciples might be ‘one’ (Jn 17: 21). And did not the same Christ will that His disciples should be marked out and distinguished from others by this characteristic, namely that they loved one another: ‘By this shall all men know that you are my disciples, if you have love one for another’ (Jn 8:35)? All Christians, they add, should be as ‘one’: for then they would be much more powerful in driving out the pest of irreligion, which like a serpent daily creeps further and becomes more widely spread, and prepares to rob the Gospel of its strength. These things and others that class of men who are known as pan-Christians continually repeat and amplify. (Pius XI. Encyclical Mortalium animos, January 6, 1928)

Certain it is that the critical issues, the thorny problems that wait upon men’s solution, have remained the same for almost twenty centuries. And why? Because the whole of history and of life hinges on the person of Jesus Christ. Either men anchor themselves on Him and His Church, and thus enjoy the blessings of light and joy, right order and peace; or they live their lives apart from Him; many positively oppose Him, and deliberately exclude themselves from the Church. The result can only be confusion in their lives, bitterness in their relations with one another, and the savage threat of war. (John XXIII. Address on the occasion of the solemn opening of Vatican Council II, October 11, 1962)

He [St. Paul] shows that the first motive for which they should be careful to abstain from eating the offerings immolated to the idols: Holy Communion; whereby what he is going to say he submits to their judgment, he shows, in the second place what it means to make ourselves one with Christ by means of Eucharistic Communion; and thirdly, he proves that this is so, that effectively we are one in his mystical body. […] He reasoning is, then, of this nature: just as he who drinks the chalice of the Lord becomes one with him, in the same way, he who drinks the chalice of the demons becomes one with them. But if there is something that above all should be fled from, it is union with the demons. (Saint Thomas Aquinas. Commentary on the First Epistle to the Corinthians, cap. 10, lec. 4: 1 Cor 10:14-17 – rep. Pet. Tar.)

There are such men confessing themselves to be Christians, and admitting the crucified Jesus to be both Lord and Christ, yet not teaching His doctrines, but those of the spirits of error, […] There are, therefore, and there were many, my friends, who, coming forward in the name of Jesus, taught both to speak and act impious and blasphemous things; […] with whom we have nothing in common, since we know them to be atheists, impious, unrighteous, and sinful, and confessors of Jesus in name only, instead of worshippers of Him. Yet they style themselves Christians, just as certain among the Gentiles inscribe the name of God upon the works of their own hands, and partake in nefarious and impious rites. (Saint Justin of Rome. Dialogue of Justin with Trypho, Ch. 35)

No one is indeed to be compelled to embrace the faith against his will; but by the severity, or one might rather say, by the mercy of God, it is common for treachery to be chastised with the scourge of tribulation. Is it the case, because the best morals are chosen by freedom of will, that therefore the worst morals are not punished by integrity of law? But yet discipline to punish an evil manner of living is out of the question, except where principles of good living which had been learned have come to be despised. If any laws, therefore, have been enacted against you, you are not thereby forced to do well, but are only prevented from doing ill. For no one can do well unless he has deliberately chosen, and unless he has loved what is in free will; but the fear of punishment, even if it does not share in the pleasures of a good conscience, at any rate keeps the evil desire from escaping beyond the bounds of thought. (Saint Augustine. Answer to Petilian the Donatist, Book III, no. 184)

First, let us examine that liberty in individuals which is so opposed to the virtue of religion, namely, the liberty of worship, as it is called. This is based on the principle that every man is free to profess as he may choose any religion or none. (…) And if it be asked which of the many conflicting religions it is necessary to adopt, reason and the natural law unhesitatingly tell us to practice that one which God enjoins, and which men can easily recognize by certain exterior notes, whereby Divine Providence has willed that it should be distinguished, because, in a matter of such moment, the most terrible loss would be the consequence of error. Wherefore, when a liberty such as We have described is offered to man, the power is given him to pervert or abandon with impunity the most sacred of duties, and to exchange the unchangeable good for evil; which, as We have said, is no liberty, but its degradation, and the abject submission of the soul to sin. (Leo XIII. Encyclical Libertas praestantissimum, June 20, 1888)

For you well know, venerable brethren, that at this time men are found not a few who, applying to civil society the impious and absurd principle of ‘naturalism,’ as they call it, dare to teach that ‘the best constitution of public society and (also) civil progress altogether require that human society be conducted and governed without regard being had to religion any more than if it did not exist; or, at least, without any distinction being made between the true religion and false ones.’ And, against the doctrine of Scripture, of the Church, and of the Holy Fathers, they do not hesitate to assert that […] ‘liberty of conscience and worship is each man’s personal right, which ought to be legally proclaimed and asserted in every rightly constituted society; and that a right resides in the citizens to an absolute liberty, which should be restrained by no authority whether ecclesiastical or civil, whereby they may be able openly and publicly to manifest and declare any of their ideas whatever, either by word of mouth, by the press, or in any other way.’ But, while they rashly affirm this, they do not think and consider that they are preaching ‘liberty of perdition’; (Saint Augustine, epistle 105, 166) and that ‘if human arguments are always allowed free room for discussion, there will never be wanting men who will dare to resist truth, and to trust in the flowing speech of human wisdom; whereas we know, from the very teaching of our Lord Jesus Christ, how carefully Christian faith and wisdom should avoid this most injurious babbling. (Pius IX. Encyclical Quanta cura, December 8, 1864)

For it is false that the civil liberty of every cult, and likewise, the full power granted to all of manifesting openly and publicly any kind of opinions and ideas, more easily leads to the corruption of the morals and minds of the people, and to the spread of the evil of indifferentism. (Denzinger-Hünermann 2979. Pius IX, Syllabus of Errors: Errors which are related to Modern Liberalism)

As all who offer themselves are received whatever may be their form of religion, they thereby teach the great error of this age-that a regard for religion should be held as an indifferent matter, and that all religions are alike. This manner of reasoning is calculated to bring about the ruin of all forms of religion, and especially of the Catholic religion, which, as it is the only one that is true, cannot, without great injustice, be regarded as merely equal to other religions. (Leo XIII. Encyclical Humanum genus, April 20, 1884)

Therefore, dear ones, from those [heretics of] which we are speaking, flee from them as a deadly poison, condemn them, withdraw from them and, if adverted by you, they don’t wish to correct themselves, avoid conversation with them because as it is written. ‘…their talk will eat its way like gangrene’ (2Tim 2:17). (Leo I, the Great. Sermon 96 against the heresy of Eutyches, 3)

Once we have attained the truth in its fullness, integrity, and purity, unity should pervade our minds, hearts, and actions. For there is only one cause of discord, disagreement, and dissension: ignorance of the truth, or what is worse, rejection of the truth once it has been sought and found. It may be that the truth is rejected because of the practical advantages which are expected to result from false views; it may be that it is rejected as a result of that perverted blindness which seeks easy and indulgent excuses for vice and immoral behavior. All men, therefore, private citizens as well as government officials, must love the truth sincerely if they are to attain that peace and harmony on which depends all real prosperity, public and private. (John XXIII. Encyclical Ad Petri cathedram, no. 20-21, June 29, 1959)