VATICAN CITY —
Not even his closest associates had advance word of the news, a bombshell that he dropped during a routine meeting of Vatican cardinals. And with no clear favorites to succeed him, another surprise likely awaits when the cardinals elect Benedict's successor next month.
"Without doubt this is a historic moment," said Cardinal Christoph Schoenborn, a protégé and former theology student of Benedict's who is considered a papal contender. "Right now, 1.2 billion Catholics the world over are holding their breath."
The Feb. 28 resignation allows for a fast-track conclave to elect a new pope, since the traditional mourning period of nine days that would follow a pope's death doesn't have to be observed. Though he will not himself vote, Benedict has hand-picked the bulk of the College of Cardinals – the princes of the church who will elect his successor – to guarantee his conservative legacy and ensure an orthodox future for the church.
"For the century to come, I think that none of Benedict's successors will feel morally obliged to remain until their death," said Paris Cardinal Andre Vingt-Trois.
Benedict said as recently as 2010 that a pontiff should resign if he got too old or infirm to do the job, but it was a tremendous surprise when he said in Latin that his "strength of mind and body" had diminished and that he couldn't carry on. As a top aide, Benedict watched from up close as Pope John Paul II suffered publicly from the Parkinson's disease that enfeebled him in the final years of his papacy. Clearly Benedict wanted to avoid the same fate as his advancing age took its toll, though the Vatican insisted the decision was not prompted by a specific malady.
The Vatican said Benedict, 85, would live in a congregation for cloistered nuns inside the Vatican, although he will be free to go in and out. Much of this is unchartered territory. The Vatican's chief spokesman, the Rev. Federico Lombardi, said he isn't even sure of Benedict's title – perhaps "pope emeritus."
As early as 2010, Benedict began to look worn out: He had lost weight and didn't seem fully engaged when visiting bishops briefed him on their dioceses. But as tired as he often seemed, he would also bounce back, enduring searing heat in Benin to bless a child and gamely hanging on when a freak storm forced him to cut short a speech in Madrid in 2011.
His 89-year-old brother, Georg Ratzinger, said doctors recently advised the pope not to take any more trans-Atlantic trips.
"He has looked very, very run down," agreed U.S. Cardinal Edwin O'Brien, who attended Monday's announcement, speaking to Sirius XM's The Catholic Channel.
Benedict emphasized that to carry out the duties of being pope, "both strength of mind and body are necessary – strengths which in the last few months, have deteriorated in me."
Benedict himself raised the possibility of resigning if he were too old or sick to continue.
"If a pope clearly realizes that he is no longer physically, psychologically and spiritually capable of handling the duties of his office, then he has a right, and under some circumstances, also an obligation to resign," Benedict said in the 2010 book "Light of the World."
But he stressed that resignation was not an option to escape a particular burden, such as the sex abuse scandal.
"When the danger is great, one must not run away. For that reason, now is certainly not the time to resign. Precisely at a time like this one must stand fast and endure the situation," he said.
The last pope to resign was Pope Gregory XII, who did so in 1415 in a deal to end the Great Western Schism, a dispute among competing papal claimants. The most famous resignation was Pope Celestine V in 1294; Dante placed him in hell for it.
There are good reasons why others haven't followed suit, primarily because of the fear of a schism with two living popes. Lombardi sought to rule out such a scenario, saying church law makes clear that a resigning pope no longer has the right to govern the church.
When Benedict was elected in 2005 at age 78, he was the oldest pope chosen in nearly 300 years. At the time, he had already been planning to retire as the Vatican's chief orthodoxy watchdog to spend his final years writing in the "peace and quiet" of his native Bavaria.
On Monday, Benedict said he plans to serve the church the rest of his days "through a life dedicated to prayer."
All cardinals under age 80 are allowed to vote in the conclave, the secret meeting held in the Sistine Chapel where cardinals cast ballots for a new pope. There are currently eligible 118 cardinals – 67 appointed by Benedict.
Benedict in 2007 passed a decree requiring a two-thirds majority to elect a pope, changing the rules established by John Paul in which the voting could shift to a simple majority after about 12 days of inconclusive balloting.
Benedict did so to prevent cardinals from merely holding out until the 12 days had passed to push through a candidate who had only a slim majority.
Contenders to be Benedict's successor include Cardinal Angelo Scola, archbishop of Milan; Schoenborn, the archbishop of Vienna, and Cardinal Marc Ouellet, the Canadian head of the Vatican's office for bishops.
Longshots include Cardinal Timothy Dolan of New York.
Although Dolan is popular and backs the pope's conservative line, being from a world superpower would probably hurt his chances.
That might also rule out Cardinal Raymond Burke, an archconservative and the Vatican's top judge, even though he is known and respected by most Vatican cardinals.
Cardinal Antonio Tagle, the archbishop of Manila, has impressed many Vatican watchers, but at 56 he is considered too young.
Cardinal Peter Kodwo Appiah Turkson of Ghana is one of the highest-ranking African cardinals at the Vatican, currently heading the Vatican's office for justice and peace, but he's something of a wild card.
There are several "papabiles" in Latin America, though the most well-known – Cardinal Oscar Andres Rodriguez Maradiaga of Honduras – is considered far too liberal to be elected by such a conservative College of Cardinals.
Whoever it is, he will face a church in turmoil: The sex abuse scandal has driven away thousands of people away from the church, particularly in Europe.
Rival churches, particularly evangelical Pentecostal groups in the developing world, pose new competition to Catholicism.
And as the pope himself has long lamented, many people in an increasingly secular world simply believe they don't need God.