Sedevacantism basically rejects the modern Popes on the basis of disagreement with the reforms of the Second Vatican Council. This means that each Pope that has embraced these reforms is not a Pope, but a heretic. "Sedevacantism is a theological position embraced by a minority of Traditionalist Catholics which holds that the Papal See has been vacant since the death of Pope Pius XII in 1958 (or, in some cases, the death of Pope John XXIII in 1963). Sedevacantists believe that Paul VI (1963–1978), John Paul I (1978), John Paul II (1978–2005) and Benedict XVI (2005-) have been neither true Catholics nor true popes, but rather notorious heretics." "As with traditionalist Catholicism in general, sedevacantism owes its origins to the rejection of the theological and disciplinary changes implemented following the Second Vatican Council (1962–1965). Sedevacantists thus reject the Council, on the basis of its documents on ecumenism and religious liberty, which they see as contradicting the traditional teachings of the Catholic Church and as denying the unique mission of Catholicism as the one true religion, outside of which there is no salvation. They also say that new disciplinary norms, such as the Mass of Paul VI, promulgated on 3 April 1969, undermine or conflict with the historical Catholic faith." "Claims used by sedevacantists to defend their position include the following: Most pre-Conciliar Catholic theologians and canon lawyers taught that it is inherently impossible for a heretic to hold the papal office. Particular provisions of Church law prevent a heretic from being elected or remaining as pope. Paul IV's 1559 Bull Cum ex apostolatus officio stipulated that a heretic cannot be elected Pope, while Canon 188.4 of the 1917 Code of Canon Law provides that a cleric who publicly defects from the Catholic faith automatically loses any office that he holds in the Church." http://en.wikipedia.org/wiki/Sedevacantism