In transforming an office with an aura of divinity into something far more human, Benedict's decision has sent shock waves through the Vatican hierarchy, who next month will elect his successor. But it has also puzzled the faithful and scholars, who wonder how a pope can be infallible one day and fallible again the next - and whether that might undermine the authority of church teaching.
Benedict stunned the world last week when he said that he would retire on Feb. 28, a decision he said he had made "in full liberty and for the good of the church." Even as the Vatican has tried to play down the confusion, saying that Canon Law provides for a clear transfer of power if a pope resigns, the implications of Benedict's act remain unclear.
"What is the status of an ex-pope?" asked Ken Pennington, a professor of ecclesiastical and legal history at the Catholic University of America in Washington. "We have no rules about that at all. What is his title? What are his powers? Does he lose infallibility?"
Some said that the very idea of a retired pope meant that the title had lost some of its luster. Other monarchies, like the British crown, have clear rules about transfer of power. Not so the Vatican.
"It's fine for the Queen Mother to be Queen Mother, but you don't really feel that Benedict can be the 'Pope Father,"' said Diarmaid MacCullough, a professor of the history of the church at Oxford University.
Although in the popular imagination, everything a pope says and writes is often perceived as infallible, in fact, papal pronouncements are only considered infallible when a pope speaks "ex cathedra," in his capacity as leader of the universal church, on questions of faith and morals. The Vatican spokesman, the Rev. Federico Lombardi, has repeatedly said that Canon Law ensures the infallibility of Benedict's successor, and that once he retires, Benedict will no longer have the authority to promulgate dogma.
Still, many remain puzzled by the larger implications.
"From a theological point of view, how can a person be considered to be infallible and not be infallible anymore?" Pennington asked.
As she stood in St. Peter's Square on Sunday to hear Benedict deliver his second-to-last Angelus message as pope, Alessandra Petrucciani said she wished he had not decided to retire.
"The pope should have stayed; the bishops and cardinals should have gone," she said, as she stood next to members of a traditionalist group who were shouting, "Stay! Stay!"
Before his decision, Benedict might have been remembered as a passive pope, a theologian who helped shape the doctrine of his beloved predecessor, Pope John Paul II, but whose own reign was marred by scandal. In stepping down, scholars say, his last act became his most revolutionary, making history and perhaps opening the door to a new era.
"The mere fact that he's resigning has permanently changed the nature of the papacy," said Eamon Duffy, a historian of Christianity at Cambridge University. "He's thought the unthinkable, done the undoable. He's broken a taboo that had last 600 years, the last 150 of which presented the pope as a religious icon, the emblem of Jesus Christ, not the leader of a global church."
Rowan Williams, a theologian who was archbishop of Canterbury from 2002 until 2012 and is now master of Magdalene College, Cambridge, said that Benedict's resignation meant that "the pope is not like a sort of God-king who goes on to the very end."
It was a statement that "the ministry of service that the bishop of Rome exercises is just that, a ministry of service and it's therefore reasonable to ask if there is a moment when somebody else should take that baton in hand," he told Vatican Radio, adding that the pope had made the role "slightly more functional, slightly less theologically top heavy."
That the supreme pontiff can pass authority to his successor at retirement rather than death inevitably introduces more ambiguity to the authority of church doctrine, some scholars say, since it calls into question the authority of the pontiff who promulgated that doctrine. "Benedict actually by resigning has introduced some cracks into that infallibility. It's bound to relativize doctrine," MacCullough said.
"That's reality hitting the Roman Catholic Church," he added. "That is actually how doctrine has always been promulgated: the result of accidents, unexpected results, contingency, context, things that aren't said. That's how things have been in Christianity right from the start."
Although its origins go back centuries, the notion of papal infallibility was effectively codified at the First Vatican Council, a meeting of church officials in the 1860s. At a time when other European monarchies were ceding more power to the mechanisms of representative democracy, papal infallibility became a kind of consolation prize for the Vatican losing its temporal powers.
In fact, the invocation of papal infallibility "ex cathedra" has only occurred twice in the modern era: In 1854, when Pope Pius IX promulgated the doctrine of the Immaculate Conception, that Mary was without original sin. And in 1950, Pope Pius XII pronounced the doctrine of the Assumption of the Virgin, that Mary had been assumed into heaven, body and spirit. The church has not ruled on whether the Virgin Mary died before she was assumed into heaven.
"If after March 1, Benedict XVI loses his head and writes that he declares in an infallible way that the Virgin Mary died before being assumed into heaven, this won't be an infallible decision, because he's no longer doing it as pastor of the universal church," said Philip Goyret, a professor of ecclesiology at the Rome's Santa Croce University, a private Catholic university run by Opus Dei. "It will be his personal opinion."
"But he's a very intelligent person and will never do that," Goyret added.
Although the Vatican has tried to play down concerns, experts and prelates worry what it will mean to have two popes alive at the same time, and both living inside the Vatican.
"It's completely uncharted waters," said Andrea Tornielli, a Vatican expert for the Turin daily La Stampa and Vatican Insider. "They say they're calm about it, but it's not easy to say what the role of the new pope will be. Will the new pope be able to create new decisions that go against those of Benedict? It's a question."
Others say that if he were to leave the Vatican, having the former pope in a different city might lead to more confusion, if the faithful perceived him to preside in a different center of power, and made pilgrimages to see him.
Assuming Benedict stays at the Vatican, as has been announced, "I can imagine these unhappy Catholics going to the old pope and saying, 'What do you think about that?"' Pennington said. "I think that this would raise serious issues of where authority and where infallibility and where the truth in the church lies."
The Vatican has acknowledged some of the confusion.
"For many it's still a surprise," the Vatican spokesman, Lombardi, said last week. "There's a lot of reflection on the significance of the decision and what this implies for the church and for the Roman Curia."
Elisabetta Povoledo contributed reporting.