BBC: The Pope has resigned because he felt he was no longer up to the demands the office made on him.
That hasn't happened in 600 years.
In 1294 the hermit Pietro da Morrone, elevated to the papacy with the title of Celestine V because the cardinals couldn't agree on anyone else, felt likewise after only six months in the job, and gave up.
He wanted to return to his hermitage, but Boniface VIII, his successor, thought it wiser to lock him up in a convenient castle for the rest of his life, fearing he might become a rallying-point for the disaffected.
And, as it turned out, there was no shortage of disaffection during Boniface's pontificate.
One of the arguments marshalled by Boniface's many enemies was that, because popes could not resign, he wasn't the legitimate heir to St Peter.
Electing an antipope?
That may have been a long time ago but the same arguments are beginning to appear.
Two distinguished Italian theologians have called on Benedict XVI to withdraw his resignation, one arguing he ought not to resign, the other claiming a pope cannot resign.
In the latter case, when the cardinals proceed to elect a successor they are, according to Enrico Maria Radaelli, electing an antipope, an impostor on the chair of St Peter. » | Michael Walsh * | Papal historian | Thursday, February 28, 2013
* Michael Walsh is a papal historian and author of several books about the Papacy, including The Popes: 50 celebrated occupants of the throne of St Peter
