The Antipope reference article from the English Wikipedia on 24-Jul-2004
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Antipope

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In the case of certain periods of turbulence in the Roman Catholic Church, elections later determined to have been invalid have set up claimants to the Papacy, and usually in opposition to a specific pope. A person so chosen is known as an antipope. The earliest of these, Hippolytus, was elected in protest against Pope Callixtus I by a schismatic group in the city of Rome in the 3rd century. Hippolytus ended his life, however, in exile during Roman imperial persecution in the mines on the island of Sardinia in the company of Callixtus' successor Pope Pontian, and was reconciled to the Catholic Church.

The late 14th and early 15th century saw a series of rival popes elected, one line of which is counted by the Roman Catholic Church as popes and the other as antipopes. The scandal of multiple claimants added to the demands for reform that produced the Protestant Reformation at the turn of the 16th century. See Western Schism, Antipope Benedict XIII.

It would not necessarily have been evident, during periods when two (or three) rival claimants existed, which was the antipope, and which was the pope, and the clear-cut distinctions made between them in retrospect can give a false sense that certainty existed among their contemporaries. Supporters might offer assistance to a given candidate, but could not know which would be determined to have been an antipope, and which the pope, until events had run their course.

There had not been an antipope since 1449, until 1978, when Antipope Gregory XVII was self-declared. Other schisms like the Church of England are controlled by lay sovereigns who do not want to have an ecclesiastical rival or begin like the Old Catholic Church in a rejection of a primary dogma of the papacy.

A few breakaway Catholics today, called sedevacantists, claim the current Popes are heretics for replacing the Tridentine Latin Mass with what they call the Novus Ordo Mass and allowing the celebration of the Mass in the vernacular. Some of them have their own popes to replace the popes they reject, but we should not consider them antipopes within the traditional sense because the number of their followers, in comparison to the size of the following of the generally accepted Popes, is minuscule. However for reasons of clarity, two such figures are described as antipopes below.

Today the act of becoming an Antipope is considered a schismatic act by the Roman Catholic Church. This would result in automatic excommunication for the person who became Antipope.

Hergenröther enumerates thirty antipopes (this does not represent an official Vatican list): 


Among modern Twentieth/Twenty-first century antipopes are: