This is from a letter I just wrote to the relative of a well-known sedevacantist, edited for clarity and length. I hope he will forgive me for reprinting part of a private letter here but I'll probably never be able to describe my feelings about the "una cum" Mass so well again.
I am now about to say something nice about SSPX, so prepare for the shock. Since it would seem to be an objective fact -- or at least you and I think so -- that these Popes are not Popes ( not just anti-Popes with invalid elections but non-Popes and non-Catholics ), then the clergy of SSPX are not really in schism when they sift and pick-and-choose. Sadly, it is those who remain in VII who bear the full brunt of schism.
Many sedevacantists say the SSPX is in schism against the "man they think is Pope." But who cares what those in SSPX think? They think wrong. How can you be in schism against a figment of your own imagination? Because if JPII was Pope, or Ratzinger is Pope, than I'm here typing this to you with Minnie Mouse on my lap, feeding me wine and cheese.
I see it a bit differently than most. Those in SSPX are not schismatics, even if they rebel against "the man they think is Pope," because he is NOT Pope. However, they may be heretics, because of their offense against the dogmas of infallibility and indefectibility.
What a terrible and blasphemous assumption they make about the Catholic Church, that it could give us Popes who say the Old Covenant is still valid and that Jews can convert people to the "light." Do you know what that means? It means Christ may as well never have come down to earth to have had the flesh torn piece by piece from His body. Here is where, if I were in SSPX, I'd be getting scared. Their offense is directly aimed at God Himself, with an implicit denial of His mission.
I know this may sound hysterical or like exaggeration, but I believe that with every "una cum" that is offered, the crown of thorns is pushed deeper down into Christ's skull.
Being una-cum with idol-worshippers is an implicit denial of the Church and thus an implicit denial of the Passion itself. The consequences of that, to put it mildly, could be serious.
The question is, how many people have a mind subtle enough to put together the syllogism I just did? I know God's mind is subtle enough ( ha ha ). But does He really expect Joe Schmo to be able to figure it out? Well, why not? God reveals his secrets to the humble. What excuse would Mr. Schmo have when SSPX itself gripes about Vatican II and its leaders with the much softer-sounding term "ecumenism," which is really a euphemism for "idolatry"?
If the Popes are "ecumenical" -- idolaters, Judaizers -- then how can they be Popes? SSPX make it sound as if worshipping strange gods is a minor fault, when it is really BLANKET APOSTASY. And the fact that so few see this can only be attributed to the shell-shock, the hypnotism, the vivid confusion spread not only by Vatican II but by SSPX.
The sedevacantist position, on the other hand, is like having smelling salts thrust under your nose.