Against the heretical Cassiciacum Thesis
See also Against the Against the SSPV.
The Cassiciacum Thesis isn’t real Sedevacantism. It claims Vatican II popes are validly elected but lack jurisdiction. Its supporters argue the Vatican II Church is still the true Church and that Novus Ordo members (unless declared heretics) are Catholic. This is false and must be rejected. A true pope has full jurisdiction; if he lacks it, he’s not pope. Heretics can’t be validly elected.