Sedevacantism is the position held by some people who identify as Catholic that the present occupier of the Holy See is not truly the pope due to the mainstream church's espousal of what they see as the heresy of modernism and that, for lack of a valid pope, the See has been vacant since the death of Pope Pius XII in 1958 or the death of Pope John XXIII in 1963.

Sedeplenist may also be used to signify the minds of Catholics who believe the Novus Ordo popes are fully fledged popes and are truly in possession of the full authority promised by Christ to Saint Peter and his successors, yet have substantially erred in the teaching of the universal ordinary magisterium on matters of faith and morals. In their perspective they believe this kind of error is possible and has occurred, so they may lawfully recognize and resist the fully seated papal authority on the most essential matters which constitute a religion: doctrine, morals, liturgy, sacramental system, code of law, and nature of government.

Modernism is the heresy of heresies, because it carries within it all previous heresies, being as it is a direct, frontal assault upon faith and all doctrine and dogma. It is liberal theological opinions developed in the late 19th and early 20th centuries. The term came to prominence in Pope Pius X 's 1907 encyclical Pascendi dominici gregis, which synthesizes and condemns modernism as embracing every heresy.