I am not a sedevacantists, I do not condone sedevacantism. But...
"Any baptized person who, while retaining the name of Christian, obstinately denies or doubts any of the truths proposed for belief by the divine and Catholic faith, is a heretic;"
What dogma do sedevacantists deny?
If it is heresy to hold that a certain man is an anitpope, then why didn't the supporters of the antipopes have to recant their heresy after the Great Western Schism was resolved?
They actually deny several dogma's but I will limit myself to listing the most obvious ones as I really don't like discussing these people.
According to Christ's ordinance, Peter is to have sucessors in his Primacy over the whole Church and for all time. (De fide.)
The successors of Peter in the Primacy are the bishops of Rome. (De fide.)
The Pope possesses full and supreme power of jurisdiction over the whole Church, not merely in matters of faith and morals, but also in Church discipline and in the government of the Church. (De fide.)
De fide: of faith: used to designate doctrines held to be revealed by God and so requiring the
unconditional assent of faith by all
source: Fundamentals of Catholic Dogma - Dr. Ludwig Ott
Nihil Obstat: Jeremiah J. O'Sullivan, D.D -Censor Deputatus
Imprimatur: Cornelius, Ep.Corgagiensis et Ap. Adm. Rossessis.
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Ever learning, and never attaining to the knowledge of the truth. Now as Jannes and Mambres resisted Moses, so these also resist the truth, men corrupted in mind, reprobate concerning the faith. But they shall proceed no farther; for their folly shall be manifest to all men, as theirs also was. 2 Timothy 3:7-9