An antipope is someone who makes a false claim to be the pope based on a process of election, installment or even self-appointment that is contrary to the Church’s laws. Historians typically recognize as antipopes only those false claimants with a significant following, such as large numbers of the faithful, powerful political backing or some portion of the College of Cardinals.
Lists of antipopes compiled by scholars vary because questions have arisen in particular cases over how to harmonize historical criteria with those of theology and canon law. The Annuario Pontificio (Pontifical Yearbook), the annual directory of the Holy See, lists 37 antipopes, beginning with Hippolytus, whose false claim to St. Peter’s throne lasted from A.D. 217 to 235, and ending with Felix V (1439-1449).
Antipopes in the early Church were typically promoted by rival ecclesiastical parties in Rome. In later centuries, they were more often puppets of secular rulers attempting to undermine or co-opt papal power. A few, however, had broad enough international support within the Church itself to become serious rivals to the popes they opposed.