An antipope is a person who makes a widely accepted claim to be the lawful Pope, in opposition to the Pope recognized by the Roman Catholic Church. Antipopes are typically those supported by a fairly significant faction of cardinals. Persons who claim to be the Pope but have few followers, such as the modern Sedevacantist antipopes, are not generally counted as antipopes (though they technically are), and therefore ignored for regnal numbering.
In several cases it is hard to tell which was, in fact, the lawful Pope and which the antipope.
In its list of the Popes, the Holy See's annual directory, Annuario Pontificio, attaches to the name of Pope Leo VIII (963-965) the following note: "At this point, as again in the mid-eleventh century, we come across elections in which problems of harmonizing historical criteria and those of theology and canon law make it impossible to decide clearly which side possessed the legitimacy whose factual existence guarantees the unbroken lawful succession of the Successors of Saint Peter. The uncertainty that in some cases results has made it advisable to abandon the assignation of successive numbers in the list of the Popes." In all cases it is clear that whichever was the Pope, the other was an antipope, since the claim of each was in fact widely accepted.
Hippolytus (d. 235) is commonly recognized as the earliest antipope, as he protested against Pope Callixtus I and headed a separate group within the Roman Church. Hippolytus was later reconciled to Callixtus's second successor Pope Pontian, when both were condemned to the mines on the island of Sardinia. He has been canonized by the Church. Whether two or more persons have been confused in this account of Hippolytus,[1] and whether Hippolytus actually declared himself to be the Bishop of Rome, remains unclear, especially since no such claim is found in the extant writings attributed to him.
clipped from en.wikipedia.org
