The Athanasian Creed
http://www.newadvent.org/cathen/02033b.htmWhosoever will be saved, before all things it is necessary that he hold the Catholic Faith. Which Faith except everyone do keep whole and undefiled, without doubt he shall perish everlastingly.
Pius IX - 1849
Nostis Et Nobiscuм
On the Church in the Pontifical States
https://www.papalencyclicals.net/pius09/p9nostis.htm10. In particular, ensure that the faithful are deeply and thoroughly convinced of the truth of the doctrine that the Catholic faith is necessary for attaining salvation.
Mirari Vos
On Liberalism and Religious Indifferentism
Gregory XVI - 1832
https://www.papalencyclicals.net/greg16/g16mirar.htm13. 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”[16] may those fear who contrive the notion that the safe harbor of salvation is open to persons of any religion whatever.
They should consider the testimony of Christ Himself that “those who are not with Christ are against Him,”[17] and that they disperse unhappily who do not gather with Him. Therefore “without a doubt, they will perish forever, unless they hold the Catholic faith whole and inviolate.”
Pope Gregory XVI - 1832
Summo Iugiter Studio, On Mixed Marriages
https://www.papalencyclicals.net/Greg16/g16summo.htm2. Therefore, guided by the example of Our predecessors, We are grieved to hear reports from your dioceses which indicate that some of the people committed to your care freely encourage mixed marriages.
Furthermore, they are promoting opinions contrary to the Catholic faith:
[ . . . ]
Finally some of these misguided people attempt to persuade themselves and others that men are not saved only in the Catholic religion, but that even heretics may attain eternal life.
Pius IX
On Promotion of False Doctrines, 1863
http://www.papalencyclicals.net/pius09/p9quanto.htm7. Here, too, our beloved sons and venerable brothers, it is again necessary to mention and censure a very grave error entrapping some Catholics who believe that it is possible to arrive at eternal salvation although living in error and alienated from the true faith and Catholic unity. Such belief is certainly opposed to Catholic teaching. There are, of course, those who are struggling with invincible ignorance about our most holy religion. Sincerely observing the natural law and its precepts inscribed by God on all hearts and ready to obey God, they live honest lives and are able to attain eternal life by the efficacious virtue of divine light and grace. Because God knows, searches and clearly understands the minds, hearts, thoughts, and nature of all, his supreme kindness and clemency do not permit anyone at all who is not guilty of deliberate sin to suffer eternal punishments.
19.
(. . .)
Let us pray that the errant be flooded with the light of his divine grace, may turn back from the path of error into the way of truth and justice and, experiencing the worthy fruit of repentance, may possess perpetual love and fear of his holy name.
Leo XIII
On Mission Societies, 1880
http://www.papalencyclicals.net/leo13/l13mis.htm6.
[ . . . ]
Do men like these pour forth their prayers to God that in His mercy he may bring to the Divine light of the Gospel by His victorious grace the people sitting in the darkness?
Pius IX
Allocution to the cardinals on the Consistory of the 17th of December, 1847:
The life of Pope Pius IX and the great events in the history of the Church during his pontificate by John Gilmary Shea, published 1877, pgs. 97 - 103
https://archive.org/details/TheLifeOfPopePiusIX1877It is assuredly not unknown to you, venerable brethren, that in our times many of the enemies of the Catholic faith especially direct their efforts toward placing every monstrous opinion on the same level with the doctrine of Christ, or of confounding it therewith, and so they try more and more to propagate that impious system of the indifference of religions.
But quite recently, we shudder to say it, men have appeared who have thrown such reproaches upon our name and apostolic dignity, that they do not hesitate to slander us, as if we shared in their folly and favored the aforesaid most wicked system. (. . .) as to suppose that not only the sons of the Church, but that the rest also, however alienated from Catholic unity they may remain, are alike in the way of salvation, and may arrive at everlasting life." We are at a loss from horror to find words to express our detestation of this new and atrocious injustice that is done us.
Pope Pius XI - 1928
Mortalium Animos
On Religious Unity
https://www.papalencyclicals.net/pius11/p11morta.htm13.
[ . . . ]
We desire that Our children should also know, not only those who belong to the Catholic community, but also those who are separated from Us: if these latter humbly beg light from heaven, there is no doubt but that they will recognize the one true Church of Jesus Christ and will, at last, enter it, being united with us in perfect charity.
Saint John Eudes
Man's Contract with God in Baptism
pages 49 - 52
https://archive.org/details/MansContractWithGodInBaptism/page/n45/mode/2upThat you may have a true faith in those things which God has revealed, it is necessary that you should believe in the Catholic Church, in which alone you can learn with certainty what God has revealed. For this reason, after you have been asked if you believe in God, you are also asked if you believe in the Catholic Church.
Certainly those who do not believe in the Catholic Church cannot have divine faith in the mysteries which they believe, but only natural and human faith; a faith of their own fancy, founded on the light of their own judgment, subject to error, and not on the promises of Jesus Christ. The Catholic Church alone possesses these promises, and on her testimony alone rests the foundation of Christian faith. As she possesses the divine promises for all days, even to the end of ages, there can be no reason to doubt whatever she proposes to our belief.
Thank God for having given you the precious gift of faith, and having made you a child of the holy Catholic Church, which is the faithful repository of the truths of salvation, and which all Christians are obliged to acknowledge as the true Church. In saying, “I believe in the holy Catholic Church’ you united yourself inseparably to this holy mother; you believe, without hesitation, all that she proposes, as proposed to you by Jesus Christ himself, who is ever with her in her instructions. Reject, then, with horror, everything at variance with her teachings, and regard it as an error calculated to endanger your faith.
However ignorant you may be, you have the true faith if you believe, without exception, all the holy Catholic Church believes and teaches; on the other hand, however learned you may be, you lose the gift and the virtue of faith if you reject any doctrine which she teaches; for her faith is your rule. “As there is but one faith,” says St. Paul, “to wish to divide it, is to destroy it.” Heretics not only differ from the Church in faith, but they also differ amongst themselves, a proof that they have not the true faith, which is one. The holy Catholic Church never has suffered, and never will suffer, a difference of faith in regard to any article. Her faith is the same in all times, in all places, and in all her true children. Thus her faith is one and the only true faith. You should be most desirous to preserve the faith in all its purity, since without it, it is impossible to do anything which merits Heaven. “Without faith it is impossible to please God.” Those who do not possess it may practice all the moral virtues, justice, sobriety, chastity, alms-deeds, prayers, mortification; and not only is this the case with heretics, but it is a truth which should be borne in mind, that these good actions, unless they have faith for their principle, will never merit Heaven for them. The law of Moses, all holy as it was, could save only those who observed it through faith.
When, therefore, you observe that those who believe not in the Church, practice some good works, offer many prayers, and lead an austere life, do not believe that they are on this account in the way of salvation, unless they have true faith; you commit an ENORMOUS SIN if you believe that they can be saved outside of the Church; that they can have faith without believing in her, or that they can be saved without faith.