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 Pope Callixtus I. Hippolytus was reconciled to Callixtus's second successor, Pope Pontian, and both he and Pontian are honoured as saints by the Catholic Church with a shared feast day on 13 August. Whether two or more persons have been confused in this account of Hippolytus and whether Hippolytus actually declared himself to be the Bishop of Rome, remains unclear, since no such claim by Hippolytus has been cited in the writings attributed to him.
Eusebius quotes from an unnamed earlier writer the story of Natalius, a 3rd-century priest who accepted the bishopric of the Adoptionists, a heretical group in Rome. Natalius soon repented and tearfully begged Pope Zephyrinus to receive him into communion.
Novatian (d. 258), another third-century figure, certainly claimed the See of Rome in opposition to Pope Cornelius, and if Natalius and Hippolytus were excluded because of the uncertainties concerning them, 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 (anti-kings) in Germany to overcome a particular emperor.
The Western Schism—which began in 1378, when the French cardinals, claiming that the election of Pope Urban VI was invalid, elected antipope Clement VII as a rival to the Roman Pope—led eventually to two competing lines of antipopes: the Avignon line (Clement VII took up residence in Avignon, France), and the Pisan line. The Pisan line, which began in 1409, was named after the town of Pisa, Italy, where the (Pisan) council had elected antipope Alexander V as a third claimant. To end the schism, in May 1415, the Council of Constance deposed antipope John XXIII of the Pisan line. Pope Gregory XII of the Roman line resigned in July 1415. In 1417, the Council also formally deposed antipope Benedict XIII of Avignon, but he refused to resign. Afterwards, Pope Martin V was elected and was accepted everywhere except in the small and rapidly diminishing area that remained faithful to Benedict XIII. The scandal of the Western Schism created anti-papal sentiment and fed into the Protestant Reformation at the turn of the 16th century.