https://en.wikipedia.org/wiki/Antipope https://en.wikipedia.org/wiki/Antipope "At this point, as again in the mid-11th century, we come across elections in whi

https://en.wikipedia.org/wiki/Antipope https://en.wikipedia.org/wiki/Antipope "At this point, as again in the mid-11th century, we come across elections in which problems of harmonising historical criteria and those of theology and canon law make it impossible to decide clearly which side possessed the legitimacy whose factual existence guarantees the unbroken lawful succession of the successors of Saint Peter. The uncertainty that in some cases results has made it advisable to abandon the assignation of successive numbers in the list of the popes." That is allegedly the official position - one must have faith in an unbroken line of Popes back to St. Peter, but practically speaking even the official compilers of The List of Popes aren't sure which to include.