The Vatican, which seemed caught off guard by the Pope's announcement, said cardinals from around the world should begin deliberations over who will be the next pope by mid-March.
Benedict, 85, announced his resignation in Latin during a meeting with cardinals Monday morning, and the news was broadcast on Vatican Radio's website shortly after.
"After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry," the Pope said.
Benedict said his strength, over the last few months, "has deteriorated in me to the extent that I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me."
The Pope's older brother, Georg Ratzinger, said Benedict had been advised by his doctors not to make any more transatlantic trips, the German dpa news agency reported. He also said his brother was having increasing difficulty walking.
"At this age, my brother wants more rest," Ratzinger, 89, told dpa.
The last pontiff to quit in office was Pope Gregory XII, who stepped down involuntarily in 1415 to end a schism in the church. The last time a pope stepped down voluntarily was all the way back in 1294, when Celestine V quit after only five months because he was overwhelmed by the demands of the post.
Benedict's announcement caught even those cardinals closest to him off-guard.
There will be no pontiff until a successor is chosen, the Vatican said. That selection process is expected to take place in mid-March.
The three Canadian cardinals who will be part of the conclave to pick a new pontiff are:
- Cardinal Marc Ouellet, head of the Vatican's office for bishops.
- Cardinal Thomas Collins, archbishop of Toronto.
- Cardinal Jean-Claude Turcotte, retired archbishop of Montreal.
Any cardinal under age 80 can vote in the selection of a new pontiff. They will be sequestered in Vatican City for a conclave that must be held between 15 and 20 days after Benedict's last day. Voting will be conducted in secret, and the ballots are burned following each round of balloting. A two-thirds majority is required to elect a pope. White smoke indicates that a new pontiff has been selected, while black smoke indicates no decision has been reached.
Cardinal Ouellet, former archbishop of Quebec, is already being mentioned in media reports as being on the list of possible successors to Benedict.
Cardinal Collins, speaking after noon mass in Toronto, said the news of Benedict's resignation came as a surprise even though the Pope had talked in the past about leaving when he felt he could no longer fulfil his mission.
"Nobody at all was expecting it this morning," he said. "I think it was just quite a shock."
Cardinal Collins would not comment on the speculation that Ouellet could be among the frontrunners to succeed Benedict, saying it is too early to consider who will replace the pontiff. He said now is the time for the Catholic Church to reflect on what it needs, not who should lead.
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Oldest Pope in centuries
Formerly known as Cardinal Joseph Ratzinger, Benedict, who was born in Germany, was chosen Pope by cardinals on April 19, 2005, following the very popular Pope John Paul II.
At the time of his selection as Pope, Benedict was 78 years old, making him the oldest pontiff elected in roughly three centuries.
Reaction to the resignation announcement poured in from around the world.
Prime Minister Stephen Harper said he and wife, Laureen, were shocked to hear the news.
"He has been a faithful steward, and will be missed," Harper said in statment, adding that Benedict "will always have a special place in the hearts of Canadians."
During Benedict's tenure, two Canadians were canonized: Saint Brother André Bessette and Saint Kateri Tekakwitha, and Collins was elevated to the College of Cardinals.
Neil MacCarthy, a spokesman for the Archdiocese of Toronto, said he was surprised but understands Benedict's unexpected resignation. He called the decision responsible, given the Pope's age.
MacCarthy said he doesn't believe declining church membership or sex abuse scandals led to the resignation.
Archbishop of Canterbury Justin Welby, spiritual head of the Anglican Communion, praised Benedict's courage.
"It was with a heavy heart but complete understanding that we learned this morning of Pope Benedict's declaration of his decision to lay down the burden of ministry as Bishop of Rome, an office which he has held with great dignity, insight and courage," Welby said.
Israel's Chief Rabbi Yona Metzger praised Benedict's efforts at outreach between religions.
"During his period [as Pope] there were the best relations ever between the church and the chief rabbinate and we hope that this trend will continue," Metzger said via a spokesperson, Reuters reported.
German Chancellor Angela Merkel said Benedict "reached out his hand to Jews as well as Muslims."
"Benedict XVI is and will remain one of the most important religious thinkers of our time," she added.
A bookish Pope who did not have the charisma of his predecessor, Benedict was a conservative theologian who tried to rekindle faith in Catholics and remind the church of its traditions. Benedict's legacy will also be tied, however, to the sex abuse scandal that engulfed the Catholic Church. Thousands of people around the world came forward to claim abuse at the hands of Catholic clergy.
Benedict met with victims and prayed with them, but he never admitted any personal failings in handling abuse cases during the 24 years that he headed the Congregation for the Doctrine of the Faith. He also never took action against bishops who ignored or failed to deal with abusive priests.
More recently, Benedict was stung by a scandal involving his personal butler, who took the pontiff's personal papers and gave them to a journalist.
Immediately following his resignation, Benedict will go to Castel Gandolfo, the papal summer retreat south of Rome. From there, he will go to live in a cloistered monastery. Benedict said he would spend his remaining years serving the church "through a life dedicated to prayer."