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God forbid the SSPX ever does.

While you oversimplify the principle (it's not about resistance but about the indefectibility of the Church), this is in fact correct.  Conservative Novus Ordites and Sedevacantists adhere to the same Major, the eminently Catholic principle that legitimate Catholic Magisterium and the Public Worship of the Church cannot become corrupt.  This principle has only been taught by every Pope, Father, Doctor, and theologian throughout the entire history of the Catholic Church ... and has been denied only by Old Catholics, Eastern Orthodox, and Protestants prior to the creation of mainstream "R&R" theology.

By R&R I refer to some modern types who claim falsely to be heirs of Archbishop Lefebvre.  Archbishop Lefebvre himself upheld the principle of the Major that the Papacy is protected by the Holy Spirit from corrupting the Church.  He simply questioned the Minor in terms of how this could have come about.  So those who try to defend their Old Catholic position as being that of Lefebvre are slandering him.

I doubt Siscoe or Salza would screw up very simple Latin, and mistake nulla salus for nullo salus, as I read in one former post.  Salus is a feminine noun and nulla is the only option.  This is basic, junior Latin.

But as for Invincible Ignorance, unless it has escaped me, I have never found that the Church has defined the subject of invincible ignorance.  I know some theologians have theorized as to what it may or may not mean, but we throw this term around as though it is a carte blanche ticket into heaven.  Fr Wathen asked the question many years ago, Why is it that these pygmies in Swahililand, Australia, can know about Coca Cola, but they somehow cannot know about the Catholic Church?