Perhaps the truth is between two extremes.
On one side is the denial of the Athanasian Creed, and the teaching of the popes, as if there were souls who could possibly be saved without turning to the Catholic faith, remaining in their ignorance, as if God would not send them a preacher or even an internal inspiration, if necessary, to teach them the truths of the Catholic faith.
On the other side is a condemnation of the positions held by St. Thomas and St. Alphonsus, and St. Bellarmine, as if it were not lawful to hold positions held by the doctors of the Church, positions which have yet to be condemned by the Church.
The likes of Bishop Fellay and Bishop Sanborn ought to be bombarded with the teaching of St. Thomas, the Athanasian Creed, and that of the popes.
St. Thomas Aquinas
Quaestiones disputatae de veritate
Question Fourteen: Faith
ARTICLE XI: In the eleventh article we ask: Is it necessary to believe explicitly?
http://www.clerus.org/bibliaclerusonline/en/g3i.htm1. We should not posit any proposition from which an untenable conclusion follows. But, if we claim that explicit belief is necessary for salvation, an untenable conclusion follows. For it is possible for someone to be brought up in the forest or among wolves, and such a one cannot have explicit knowledge of any matter of faith. Thus, there will be a man who will inevitably be damned. But this is untenable. Hence, explicit belief in something does not seem necessary.
Answers to Difficulties
1. Granted that everyone is bound to believe something explicitly, no untenable conclusion follows even if someone is brought up in the forest or among wild beasts. For it pertains to divine providence to furnish everyone with what is necessary for salvation, provided that on his part there is no hindrance. Thus, if someone so brought up followed the direction of natural reason in seeking good and avoiding evil, we must most certainly hold that God would either reveal to him through internal inspiration what had to be believed, or would send some preacher of the faith to him as he sent Peter to Cornelius (Acts 10:20).
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.