9 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." 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? And in what manner, We ask, can men who follow contrary opinions, belong to one and the same Federation of the faithful? For example, those who affirm, and those who deny that sacred Tradition is a true fount of divine Revelation; those who hold that an ecclesiastical hierarchy, made up of bishops, priests and ministers, has been divinely constituted, and those who assert that it has been brought in little by little in accordance with the conditions of the time; those who adore Christ really present in the Most Holy Eucharist through that marvelous conversion of the bread and wine, which is called transubstantiation, and those who affirm that Christ is present only by faith or by the signification and virtue of the Sacrament; those who in the Eucharist recognize the nature both of a sacrament and of a sacrifice, and those who say that it is nothing more than the memorial or commemoration of the Lord's Supper; those who believe it to be good and useful to invoke by prayer the Saints reigning with Christ, especially Mary the Mother of God, and to venerate their images, and those who urge that such a veneration is not to be made use of, for it is contrary to the honor due to Jesus Christ, "the one mediator of God and men." 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. Those, who are unhappily infected with these errors, hold that dogmatic truth is not absolute but relative , that is, it agrees with the varying necessities of time and place and with the varying tendencies of the mind, since it is not contained in immutable revelation, but is capable of being accommodated to human life. Besides this, in connection with things which must be believed, it is nowise licit to use that distinction which some have seen fit to introduce between those articles of faith which are fundamental and those which are not fundamental, as they say, as if the former are to be accepted by all, while the latter may be left to the free assent of the faithful: for the supernatural virtue of faith has a formal cause, namely the authority of God revealing, and this is patient of no such distinction. For this reason it is that all who are truly Christ's believe, for example, the Conception of the Mother of God without stain of original sin with the same faith as they believe the mystery of the August Trinity, and the Incarnation of our Lord just as they do the infallible teaching authority of the Roman Pontiff, according to the sense in which it was defined by the Ecumenical Council of the Vatican. Are these truths not equally certain, or not equally to be believed, because the Church has solemnly sanctioned and defined them, some in one age and some in another, even in those times immediately before our own? Has not God revealed them all? For the teaching authority of the Church, which in the divine wisdom was constituted on earth in order that revealed doctrines might remain intact for ever, and that they might be brought with ease and security to the knowledge of men, and which is daily exercised through the Roman Pontiff and the Bishops who are in communion with him, has also the office of defining, when it sees fit, any truth with solemn rites and decrees, whenever this is necessary either to oppose the errors or the attacks of heretics, or more clearly and in greater detail to stamp the minds of the faithful with the articles of sacred doctrine which have been explained. But in the use of this extraordinary teaching authority no newly invented matter is brought in, nor is anything new added to the number of those truths which are at least implicitly contained in the deposit of Revelation, divinely handed down to the Church: only those which are made clear which perhaps may still seem obscure to some, or that which some have previously called into question is declared to be of faith.
Source: Mortalium Animos (Vatican.va)