An antipope () is a person who makes a significant and substantial attempt to occupy the position of Bishop of Rome and leader of the Catholic Church in opposition to the legitimately elected pope. At times between the 3rd and mid-15th centuries, antipopes were supported by important factions within the Church itself and by secular rulers. Sometimes it was difficult to distinguish which of two claimants should be called pope and which antipope, as in the case of Pope Leo VIII and Pope Benedict V. Persons who merely claim to be pope and have few followers, such as the modern conclavist antipopes, are not classified with the historical antipopes. History 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 as Clement VII moved back to Avignon, and the Pisan line. The Pisan line, which began