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The way we know with infallibly certainty that a doctrine has been revealed by God, is because an infallible teacher has infallibly taught that the doctrine has been revealed by God.
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Everyone who attends mass at a Sedevacantist sect is, by definition, manifest heretic and notorious heretic.

You start on the right foot but then twist it.  Just because the position has not been taught by an infallble teacher doesn't make it wrong.  It simply changes the theological note of the argument.

In a way, you've done PRECISELY what you condemn as Protestantism.  Based on your judgment, you declare SVs manifest heretics.  Oh the hypocrisy.

And if you were to assert that SVism is condemned by the Church (I've actually seen no explicit condemnation), you're begging the question that the Conciliar Church is the Catholic Church.

You've basically ended up hypocritically doing the same thing you condemn as Protestant heresy.

This is a perfect example the ignorance of Sedevacantists, and the false conclusions they reach by the use of their private judgement.

Mr. Ladislaus, what is de fide (Dei Filius, Vatican I) is that a doctrine is infallibly propose by the ordinary and universal magisterium when 1) all the bishops of the world agree that 2) the doctrine in question is a revealed truth that must be assented to with divine and Catholic faith.  The universality of the teaching alone does not suffice for it to be infallibly proposed; it must also be proposed definitively as revealed, for it to be infallible by the force of the ordinary and universal magisterium.

The indefectibility of the Church is related to the de fide credenda dogma of the perpetuity of Papal succession defined in the Dogmatic Constitution of the Church “Pastor aeternus.” The dogma of the indefectibility of the Church is moreover closely related to the doctrine of the permanence of the Church, and although it has not been proclaimed in an independent extraordinary decree of its own, it is considered a secondary object of the infallibility of this teaching, and as such, is a tenenda infallible teaching in its own right. The indefectibility of the Church is listed by Dr. Ludwig Ott as a sententia theologice certa (“theologically certain teaching” or Sent. certa.) dogma, meaning that it is a teaching that the Magisterium has definitively proposed. The dogma of the indefectibility of the Church has been summarized as follows: “The Church is indefectible, that is, she remains and will remain the Institution of Salvation, founded by Christ, until the end of the world” (XIV).