From James Larrabee, cited here:
"Some argue that we cannot know if the “Pope” is a formal or merely a material heretic, because only God can judge the interior, and heresy is a matter of interior rejection of Catholic doctrine, not merely exterior. It’s amazing to me that any Catholic could make this argument, but many do, among many supposedly traditional priests and laymen dedicated to defending conciliar “pontiffs.” It utterly destroys the visible nature of the Church. The question is entirely in the external forum, thus it relies solely on externally verifiable evidences, as in every other legal proceeding. If this argument were valid, no one could ever have been condemned as a heretic, or indeed could be condemned of any crime at all, either by the Church or by secular authorities, since the guilty intention essential to any crime could never be judged. The fundamental principle of reason is that people are responsible for their actions. If a criminal act is committed, it is assumed, until proven otherwise, that the person is liable for it. This is stated in canon law (2200:2):
“Posita externa legis violatione, dolus in foro externo praesumitur, donec contrarium probetur.”
"When an external violation of the law has been posited, criminal intent is presumed in the external forum, until the contrary is proved."
It is further argued that, in the case of heresy, dolus consists in the pertinacious rejection of Catholic teaching, which cannot be assumed merely from a denial of a doctrine. This is true, but pertinacity can also, and must also, be inferred from the person’s actions. That is why one or two admonitions are mentioned by St. Paul. These admonitions do not need to be formal and explicit, to judge that pertinacity is present, at least they do not need to be made by every individual. If the public evidence is sufficient for a moral certitude of pertinacity, then the conclusion follows. In the case of men who are well versed in Catholic theology, and who were well aware of their opposition to the teachings of past Popes, who heard in person the rejection and refutation of many of their errors at Vatican II, as well as having studied the condemnations of Modernism by all the Popes since Pius IX and by the Vatican Council, it cannot reasonably be denied that they were well aware of their rejection of the past teaching of the Church. So in the case of many (not necessarily all) of their teachings, pertinacity is morally certain from any intelligent consideration of the history of the Vatican revolution. In addition, the inveterate complicity of those at Rome, Wojtyla and Ratzinger most evidently, with the far more open heresy of their fellows such as Kung, Kasper, Schillebeeckx, Rahner, De Lubac, and so on ad infinitum, along with their universal toleration of any and all errors by lesser men, would alone suffice to judge pertinacity. In addition is their use of all convenient means to suppress the orthodox and destroy tradition. None of these things is compatible, to any Catholic with the least common sense and knowledge of the Faith, with an intention to profess the Catholic Faith, and essentially the same has been stated by Popes and Councils. Pope Pius VI, in the bull Auctorem Fidei, depicts the use made of ambiguity by heretics, in the Synod of Pistoia; St. Pius X, citing the Council of Constantinople, in Pascendi, refers to heretics as those who would overturn even a single one not only of the Apostolic, but even of the ecclesiastical traditions. Those engaging in ecumenism were described by Pope Pius XI as departing even from divinely revealed religion. This is all in the external forum.
[...] In addition, Vatican II did openly deny defined dogma. Religious liberty was condemned in an ex cathedra definition by Pope Pius IX, as can be read in Quanta Cura, wherein a clear formula of definition is contained (We by Our Apostolic Authority, etc.). The teaching of Vatican II is almost verbatim the contrary of what was condemned.
It seems pointless to refer to more simplistic arguments against “sedevacantism”. For example, that it is schismatic. Yet those who adhere to a FALSE Pope are the schismatics, not those who reject him."
See also:"Razing the Bastions" and the agenda of the Nouvelle Theologie modernists
Recommended resource found at Novus Ordo Watch:
Sedevacantism:The Vacancy of the Papal Throne since the Death of Pope Pius XII
