Religious Unity & Indifferentism

Defined as "The denial that the worship of God and the practice of the true religion is a duty of man." It is also commonly referred to as "other religions besides Catholicism are acceptable to God". See references from Scripture and Church teaching below:
 

References In Scripture:

• "But though we, or an angel from heaven, preach a gospel to you besides that which we have preached to you, let him be anathema. As we said before, so now I say again: If any one preach to you a gospel, besides that which you have received, let him be anathema." Galations 1:8-9
• "Go ye into the whole world, and preach the gospel to every creature. He that believeth and is baptized, shall be saved: but he that believeth not shall be condemned." Matthew 16:15
• "A man that is a heretic, after the first and second admonition, avoid: Knowing that he, that is such an one, is subverted, and sinneth, being condemned by his own judgment." Titus 3:10-11
• "If any man come to you, and bring not this doctrine, receive him not into the house nor say to him, God speed you. For he that saith unto him, God speed you, communicateth with his wicked works." 2 John 1:10-11
• "He that is not with me, is against me: and he that gathereth not with me, scattereth" Matthew 12:30
• "But he that shall deny me before men, I will also deny him before my Father who is in heaven." Mat 10:33
• "Now I beseech you, brethren, to mark them who make dissensions and offences contrary to the doctrine which you have learned, and avoid them. For they that are such, serve not Christ our Lord, but their own belly; and by pleasing speeches and good words, seduce the hearts of the innocent" Romans 16:17-18

Catholic Church Teaching on Religious Unity & Indifferentism

• "Certainly such attempts can nowise be approved by Catholics, founded as they are on that false opinion which considers all religions to be more or less good and praiseworthy, since they all in different ways manifest and signify that sense which is inborn in us all, and by which we are led to God and to the obedient acknowledgment of His rule. Not only are those who hold this opinion in error and deceived, but also in distorting the idea of true religion they reject it, and little by little turn aside to naturalism and atheism, as it is called; from which it clearly follows that one who supports those who hold these theories and attempt to realize them, is altogether abandoning the divinely revealed religion." Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "If our Redeemer plainly said that His Gospel was to continue not only during the times of the Apostles, but also till future ages, is it possible that the object of faith should in the process of time become so obscure and uncertain, that it would be necessary to-day to tolerate opinions which are even incompatible one with another? If this were true, we should have to confess that the coming of the Holy Ghost on the Apostles, and the perpetual indwelling of the same Spirit in the Church, and the very preaching of Jesus Christ, have several centuries ago, lost all their efficacy and use, to affirm which would be blasphemy." Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "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. Who then can conceive a Christian Federation, the members of which retain each his own opinions and private judgment, even in matters which concern the object of faith, even though they be repugnant to the opinions of the rest?" Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "How so great a variety of opinions can make the way clear to effect the unity of the Church We know not; that unity can only arise from one teaching authority, one law of belief and one faith of Christians. But We do know that from this it is an easy step to the neglect of religion or indifferentism and to modernism, as they call it." Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: 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" Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "For since the mystical body of Christ, in the same manner as His physical body, is one,[22] compacted and fitly joined together,[23] it were foolish and out of place to say that the mystical body is made up of members which are disunited and scattered abroad: whosoever therefore is not united with the body is no member of it, neither is he in communion with Christ its head" Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "The Catholic Church is alone in keeping the true worship. This is the fount of truth, this the house of Faith, this the temple of God: if any man enter not here, or if any man go forth from it, he is a stranger to the hope of life and salvation. Let none delude himself with obstinate wrangling. For life and salvation are here concerned, which will be lost and entirely destroyed, unless their interests are carefully and assiduously kept in mind" Encyclical "Mortalium Animos" (On Religious Unity), by Pope Pius XI in 1928
• "Now We consider another abundant source of the evils with which the Church is afflicted at present: indifferentism. This perverse opinion is spread on all sides by the fraud of the wicked who claim that it is possible to obtain the eternal salvation of the soul by the profession of any kind of religion, as long as morality is maintained. Surely, in so clear a matter, you will drive this deadly error far from the people committed to your care. With the admonition of the apostle that "there is one God, one faith, one baptism"." Encyclical On Liberalism and Religious Indifferentism, Pope Gregory XVI, 1832
• Indifferentism is also clearly condemned by name in the "SYLLABUS OF ERRORS CONDEMNED BY PIUS IX" in 1864
• To hold, therefore, that there is no difference in matters of religion between forms that are unlike each other, and even contrary to each other, most clearly leads in the end to the rejection of all religion in both theory and practice. And this is the same thing as atheism, however it may differ from it in name. Men who really believe in the existence of God must, in order to be consistent with themselves and to avoid absurd conclusions, understand that differing modes of divine worship involving dissimilarity and conflict even on most important points cannot all be equally probable, equally good, and equally acceptable to God" ON THE CHRISTIAN CONSTITUTION OF STATES, Pope Leo XIII, 1885
• "Such is the nature of Catholicism that it does not admit of more or less, but must be held as a whole or as a whole rejected: "This is the Catholic faith, which unless a man believe faithfully and firmly; he cannot be saved" (Athanas. Creed). There is no need of adding any qualifying terms to the profession of Catholicism: it is quite enough for each one to proclaim "Christian is my name and Catholic my surname," only let him endeavour to be in reality what he calls himself" Encyclical on "Appealing for Peace" by Pope Benedict XV, 1914
• "Most of the laws passed by the Church refer to the relations between its subjects and infidels in not only religious but also civil affairs. Speaking generally, the faithful are forbidden to take part in any religious rites, considered as such, of pagans, Mohammedans, or Jews, and all the more to practice them through a kind of survival of their primitive superstitions" 1917 Catholic Encyclopedia, Infidels
   

Summary

Scripture as well as the history of Catholic Church teaching clearly show religious unity, and indifferentism to be false. When Our Lord said "he that believeth not shall be condemned", it didn't mean that other beliefs were acceptable. Believing in Catholicism and another so-called religion which teaches contrary to it, is the same as believing in truth and error at the same time. The Catholic Church has never taught this and one cannot call themselves Catholic by believing in religious unity or indifferentism.