AntiPope

An antipope, Latin: antipapa, is a person who, in opposition to the one who is generally seen as the legitimately elected Pope, makes a significantly accepted competing claim to be the Pope, the Bishop of Rome and leader of the Catholic Church.

At times between the 3rd century and mid-15th century, antipopes were supported by a fairly significant faction of religious cardinals and secular kings and kingdoms. 

Hippolytus of Rome (d. 235) is commonly considered to be the earliest Antipope, as he headed a separate group within the Church in Rome against Papa Callistus Primus. Hippolytus was reconciled to Papa Pontianus, and both he and Pontianus are honoured as Saints by the Roman Catholic Church with a shared feast day on 13 August. There is some question regarding if Hippolytus actually declared himself to be the Bishop of Rome as no such claim by Hippolytus has been cited in the writings attributed to him.

Eusebius of Caesarea quotes from an unnamed earlier writer the story of one Papa Natalius, a 3rd-century priest who accepted the bishopric of a heretical group in Rome. Papa Natalius soon repented and tearfully begged Papa Zephyrinus to receive him into communion. Papa Novatian (d. 258), another 3rd-century figure, claimed the See of Rome in opposition to Papa St Cornelius, and if Antipapa Natalius and Antipapa Hippolytus were excluded because of the uncertainties concerning them, Papa Novatian could then be said to be the first Antipope.

The period in which antipopes were most numerous was during the struggles between the Popes and the Holy Roman Emperors of the 11th and 12th centuries. The emperors frequently imposed their own nominees to further their own causes. The popes, likewise, sometimes sponsored rival imperial claimants (antikings) in Germany to overcome a particular emperor.

The Western Schism— which began in 1378, when the French cardinals, claiming that the election of Papa Urbanus Sextus was invalid,  and elected Papa Clemens Septimus  led to two, and eventually three, rival lines of claimants to the papacy: the Roman line, the Avignon line, and the Pisan line. See The Western Schism page for more information.

A further confusion can be drawn from the numbering of the Popes and Antipopes. In some cases the numeration of legitimate Popes have included the numerations used by a number of Antipopes. The Antipopes who's numeration has been including in the numeration of legitimate Popes are:

Papa Felix Secundus, 355 - 365

Papa Ioannes Sextus Decimus, 997 - 998

Papa Benedictus Decimus, 1058 - 1059

Papa Alexander Quintus, 1409 - 1410