Sedevacantism is the position held by a minority of Traditionalist Catholics who claim that the Papal See has been vacant since either the death of Pope Pius XII in 1958 or Pope John XXIII in 1963. Sedevacantists believe that Paul VI (1963–1978), John Paul I (1978), John Paul II (1978–2005) and Benedict XVI (since 2005) have been neither true Catholics nor true Popes, by virtue of allegedly having espoused the heresy of Modernism, or of having otherwise denied or contradicted solemnly defined Catholic dogmas. Some of them classify John XXIII (1958–1963) also as a Modernist antipope.