Paul VI called Vatican II 'magisterium'.  He was wrong.  Popes can be wrong.  Pope Nicholas I said baptism "in the Name of Christ" is valid.  He was wrong.  He did not cease to be pope.

Feel free to ask me.  I will try my best to give an answer based on traditional Church teaching.  Or skip me, I'm fallible, and consult Church teaching yourself.

When a heresy rears its head, it is the pope's strict duty to fight it.  Honorius wrote about the very topic the contemporary heresy was concerned with, and failed to use the opportunity to fight Monothelitism.

When St. Peter went along with the demands of the Circuмcisionists, he did not say they were right, but he did not use the opportunity to clearly put down the heresy.  That is why St. Paul resisted him publicly--to his face, and Holy Scripture says St. Paul was right.  We are right to resist these popes publicly-- to the face.

NIFH: Paul VI called Vatican II 'magisterium'.  He was wrong.  Popes can be wrong.  Pope Nicholas I said baptism "in the Name of Christ" is valid.  He was wrong.  He did not cease to be pope.

• No Pope has ever been a heretic
• If a Pope were to become a manifest heretic, he would immediately cease to be Pope because he would immediately cease to be a member of the Church
• He would be deposed not by the Church, which has no authority over the Pope, but by God Himself, who has made membership in the Church dependent upon profession of the true Faith, on which the Church’s unity depends (see Pope Pius XII, Encyclical Mystici Corporis, n. 22)
• The Church’s bishops  (Note:  the ones who are not apostates) could declare the former Pope to have deposed himself — something that would enable them to remove the non-Pope
• The very idea of a heretical Pope who nevertheless remains Pope is “injurious” to the papacy and thus to Catholic dogma

"the very idea of a heretical Pope"----so don't even think about it! 

NIFH: Feel free to ask me.  I will try my best to give an answer based on traditional Church teaching.

Paul VI
"it still provided its teaching with the authority of the supreme ordinary magisterium. This ordinary magisterium, which is so obviously official, has to be accepted with docility, and sincerity by all the faithful"

(Paul VI, Audience of Jan. 12, 1966; English translation from The Pope Speaks 11, n. 2 [Spring 1966], pp. 152-154; underlining added. Italian original here.)

All those things are to be believed with divine and Catholic faith which are contained in the Word of God, written or handed down, and are proposed by the Church either by a solemn judgment or by her ordinary and universal magisterium to be believed as divinely revealed.
(Dogmatic constitution Dei Filius, chapter 3, “Concerning Faith”, Denzinger 1792)

NIFH: When St. Peter went along with the demands of the Circuмcisionists, he did not say they were right, but he did not use the opportunity to clearly put down the heresy.  That is why St. Paul resisted him publicly--to his face, and Holy Scripture says St. Paul was right.  We are right to resist these popes publicly-- to the face.

…[W]hen St. Peter compelled the Gentiles to Judaize, this was not an error of preaching but of conduct, as Tertullian suggests in his work de Praescriptionibus adversus haereticos. St. Peter did not ratify by some decree that they must Judaize, rather, he formally taught the contrary in Acts XV. Nevertheless, when he was still in Antioch, he separated himself from the dinner table of the Gentiles lest he would give offense to those recently converted to the faith from the Jєωs and by his example compelled them to Judaize in a certain measure, even Barnabas. But we do not deny that Popes can offer the occasion of erring through their own bad example, rather, we deny that they can prescribe the whole Church to follow some error ex cathedra. Moreover, the examples and doctrines of the Pontiffs are not equally pernicious to the Church, seeing that the Lord instructed them, saying: “Do what they say, but do not do what they do.”
(St. Robert Bellarmine, , vol. 2[/url], trans. by Ryan Grant [Mediatrix Press, 2016], Book IV, Ch. 8, pp. 175-176; italics given; underlining added.)[/font][/color]

To think that an Ecuмenical Council directed to all the faithful is not Magisterium ...

Materially heretical popes are a sad fact of history.  Formally heretical popes are an impossibility.  They would no longer be pope.

Vatican I spoke about submitting to universal magisterium.  Universal means:  all places and all times.  Even if the bishops teach heresy in all places today, they have not in all times.  That is not 'universal' magisterium.

We are not united in any way with formal heretics and apostates.  Solely material heretics are a different case.

Has there been an apostasy since Vatican Council II? The answer can be found in the teachings of Pope Pius XI in Mortalium Animos onFostering True Religious Unity, January 6, 1928
They presuppose the erroneous view that all religions are more or less good and praiseworthy, inasmuch as all give expression, under various forms, to that innate sense which leads men to God and to the obedient acknowledgment of His rule… to favor this opinion, therefore, and to encourage such undertakings, is tantamount to abandoning the religion revealed by God.”

Quote

Let us focus attention on the phrase “tantamount to abandoning the religion revealed by God.” This phrase is another definition of the word “apostasy.” According to Pope Pius XI, to hold to false ecuмenism and to encourage it, is equivalent to apostasy.

Since this is seriously truncated for a forum post, please see the full text here:
https://cmri.org/articles-on-the-traditional-catholic-faith/the-last-days-and-false-ecuмenism/

Vatican I spoke about submitting to universal magisterium.  Universal means:  all places and all times.  Even if the bishops teach heresy in all places today, they have not in all times.  That is not 'universal' magisterium.

We are not united in any way with formal heretics and apostates.  Solely material heretics are a different case.

St. Cyril of Alexandria echoed these same sentiments when he said, “It is therefore unlawful, and a profanation, and an act the punishment of which is death, to love to associate with unholy heretics, and to unite yourself to their communion.”

Council of Carthage in the fifth century decreed, “One must neither pray nor sing psalms with heretics, and whoever shall communicate with those who are cut off from the communion of the Church, whether clergy or layman: let him be excommunicated.”

St. Thomas Aquinas, said, “To know whom to avoid is a great means of saving our souls…Thus, the Church forbids the faithful to communicate with those unbelievers who have forsaken the faith by corrupting it, such as heretics, or by renouncing it, such as apostates.”

So holding false ecuмanism is apostasy, therefore all the signers of Vatican II apostatized.

Actually, this is the definition:


A teaching of ordinary and universal magisterium is a teaching on which all bishops (including the Pope) universally agree, and is also considered infallible. Such a teaching must also be a part of the sensus fidelium.

The question was also raised by a Cardinal, “What is to be done with the Pope if he becomes a heretic?” It was answered that there has never been such a case; the Council of Bishops could depose him for heresy, for from the moment he becomes a heretic he is not the head or even a member of the Church. The Church would not be, for a moment, obliged to listen to him when he begins to teach a doctrine the Church knows to be a false doctrine, and he would cease to be Pope, being deposed by God Himself.
If the Pope, for instance, were to say that the belief in God is false, you would not be obliged to believe him, or if he were to deny the rest of the creed, “I believe in Christ,” etc. The supposition is injurious to the Holy Father in the very idea, but serves to show you the fullness with which the subject has been considered and the ample thought given to every possibility. If he denies any dogma of the Church held by every true believer, he is no more Pope than either you or I; and so in this respect the dogma of infallibility amounts to nothing as an article of temporal government or cover for heresy.
(Abp. John B. Purcell, quoted in Rev. James J. McGovern, Life and Life Work of Pope Leo XIII [Chicago, IL: Allied Printing, 1903], p. 241)

What about notoriety by fact? It's very clear that some v2 'popes' have publicly worshipped with false religions. And also published heretical docuмents.