Sede (chair) vacante (empty) when referring to the Holy Catholic Church is a condition in which the Chair of Peter is empty or vacant. This is true when a Pope has died or resigned. However, a theory arose in the late 1960s following the Second Vatican Council that all Popes after Pius XII were heretics and under Canon Law were automatically excommunicated. Thus the Chair of Peter is vacant and has been vacant for nearly 70 years. With no direct authority leading back to the line of Apostolic succession, the Catholic Church is effectively ended.
The second part of this theory is that only a small remnant of independent Bishops and Priests remain who are legitimate Catholics. What we see in modern society as the visible Catholic Church is in fact an imposter church operating the mechanism of the lost Catholic Church.
Sedevacantism is the abandonment of all hope and is a sin against the Holy Ghost.
Those adhering to the sedevacantist position posit all manner of scholarship to defend their position. Often they call upon the 1917 Code of Canon Law, St. Robert Belarmine, and other sources to back up their claims. Usually they use "sound bites" taken out of context, clever twisting of language, and emotional appeals to make their points and hope that the reader does not take the time to research the source or consider the logic of what is being proposed. The human mind is most capable of justifying any sin. And it is so much easier if someone else has done the work for you.
Eventually they call upon other sedevacantist authors whom they have raised to near sainthood for their vast wisdom and expertise to legitimize their claims. In the end, the arguments circle back on themselves closing and reinforcing the loop.
It is difficult to debate their slippery arguments in that they diminish, ignore, and sideline anything that doesn't support their position. As the saying goes, "Its like trying to nail Jello to the wall."
Sedevacantists come in all shapes and sizes. There are independent traveling "priests" all the way to complex multi-faceted organizations which mimic the structure of the Catholic Church including assuming its names and titles. They use a similiar tactic to what the tens of thousands of honest protestant denominations have done in adopting the trappings, ceremony, titles, etc that serve their purposes.
Some of the groups may be nominally sedevacantist and try to get along with other Traditional Catholic groups, while others are hard core and condemn anyone outside of their group as traitors to Our Lord.
The following is a list of some of the groups we have encountered over the years:
SSPV (Society of St. Pius V)
CMRI (Congregation of Mary Immaculate Queen)
MHF(Most Holy Family Monastery)
In addition, there are many individuals who have claimed the papacy and elected one of their own as pope in exile.
The corporate sedevacantist structure has some similarities to modern American corporatism. In particular the need for legal structures to protect those at the top from anyone they damage, and the need to generate income to maintain the structure.
Any business has a product to sell. Take for instance a theater. They produce plays (or movies or music concerts, etc). In order to make money from the performance of the play, they need people to pay to watch. How do they get "butts in the seats?" They have a marketing arm to promote what they are selling and with a good enough sales pitch they can draw enough interest even if the play is mediocre to break even or make a profit. The better the marketers, the better the profit.
Our friends in the sedevacantist camp do the same thing. All of these groups produce a performance under the borrowed name of "Catholic Mass." They promote through word of mouth and in modern terms, internet marketing. There are numerous sites dedicated to promoting sedevacantist organizations to get "butts in the seats," as well as refill the ranks of those who have learned the truth about the sedevacantist scam and have left continuing their search for truth.
Sites we have encountered which are highly suspect at best are: