Sedevacantism is the position held by some traditional Catholics that the present occupier of the Holy See is not a valid pope due to the mainstream church's alleged espousal of modernism and that, for lack of a valid pope, the See is vacant.
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Most sedevacantists believe that the Holy See has been vacant since the death of Pope Pius XII in 1958. Others believe that there is no certainty of the vacancy until the moment before Pope Paul VI promulgated Lumen Gentium, which sedevacantists regard as heretical. Very few others regard the Holy See to be vacant since the death of Pope John XXIII in 1963, the death of Pope Paul VI in 1978, or even the death of Pope Pius X in 1914.
The term sedevacantism is derived from the Latin phrase sede vacante, which means "with the chair being vacant". The phrase is commonly used to refer specifically to a vacancy of the Holy See from the death, the resignation, fall into the insanity or the public heresy of a pope to the election of his successor.
Among those who maintain that the see of Rome, occupied by what they declare to be an illegitimate pope, was really vacant, some have chosen an alternative pope of their own, and thus in their view ended the vacancy of the see, and are known sometimes as conclavists.