An antipope is someone who claims to be the Pope, but is not recognized as a validly elected pope by the Roman Catholic Church. The only official Pope is the one elected and ratified in Rome by a council of cardinals upon the death of the last Pope. Antipopes reflect arguments and schisms within the church, some of which have historically been political in nature, while others have stemmed from disputes over religious doctrine.
The first recorded and widely recognized antipope was Hippolytus, in the third century CE. Ultimately, Hippolytus was canonized by the Catholic Church as a saint, because he reconciled with the church before his death. The last notable antipope was Felix IV, in the 1400s, illustrating the remarkable stability of the Roman Catholic Church after it managed to gain the upper hand in the Papal Schism of the early 1400s. A number of lesser antipopes have cropped up since this time, but usually in such isolated areas and with such small groups of followers that they have not been considered important.
There are a number of ways in which someone can become an antipope. At various points in history, people have simply declared themselves popes, but typically such antipopes gain little public support or attention. Most typically, an antipope arises as the result of a religious faction working in opposition to the Roman Catholic Church. During the Papal Schism, for example, rival groups in Avignon and Rome vied for control of the Papacy, and each elected its own pope.
Antipopes can also be elected by third party organizations, as also happened in the Papal Schism, when an attempt was made to reconcile the problem and a third rival to the papacy was produced instead. In some cases, antipopes have also been declared by political entities, such as kings, typically out of a desire to gain control of the church for political or social reasons.
Historically, when the Roman Catholic Church has received word of an antipope, it has often moved to excommunicate the antipope, along with his supporters. This severe penalty is designed both to punish the antipope and his faction, and to serve as a warning to would-be dissenters in the church. Rome maintains control of the Catholic Church and wishes to make this clear to Catholics all over the world; policy changes, decisions to alter the liturgy, and reforms in religious doctrine must all originate from Rome, just as the Pope himself does.