Citing failing strength of “mind and body,” Pope Benedict XVI stunned his closest aides and more than 1 billion Catholics by resigning on Monday, becoming the first pope to do so in nearly 600 years and ending the tenure of a formidable theologian who preached a gospel of conservative faith to a fast-changing world.

In keeping with his reputation as a traditionalist, Pope Benedict delivered his resignation — effective Feb. 28 — in Latin, to a private church body in Vatican City. “I have had to recognize my incapacity to adequately fulfill the ministry entrusted to me,” he said. “For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of bishop of Rome, successor of Saint Peter.”

The decision by the 85-year-old German pontiff sets up a pivotal leadership contest in the marbled halls of the Vatican that is coming sooner than observers expected. Although questions about the pope’s health have long swirled — he was occasionally filmed nodding off during Mass — he seemed committed to continuing a papacy that has divided Catholics and non-Catholics alike.

But the pope’s brother, Georg Ratzinger, also a priest, said the pontiff had informed him of his decision “months ago.”

“He has gotten tired faster and faster, and walking has become hard for him,” Ratzinger said, adding that his brother — who was born Joseph Aloisius Ratzinger and ordained a priest in the aftermath of World War II — did “the best he possibly could have done” in his role.

The conclave to choose the next pope is expected to convene in mid-March, with a new pope in place in time to preside over Easter Mass.

The pontiff departs amid a sense of crisis in the Vatican. The institution’s most recent problems involve a bevy of documents leaked by the pope’s butler, Paolo Gabriele, to Italian journalists alleging corruption and heated disputes within the Vatican walls. The church also has faced criticism for its internal bank’s failures to comply with international rules governing money laundering. The Vatican’s financial troubles escalated this year to the point where international banks temporarily suspended credit card links at the Sistine Chapel, forcing tourists to use cash.

Perhaps the greatest challenge of his papacy has emerged in the spread of clerical sex-abuse scandals from the United States into other places including Ireland and Germany, where the pope was born and served as an archbishop. Critics have urged that more bishops be held accountable, and some have raised questions about Benedict’s management of a case involving a German priest and sex offender while he was bishop of Munich in 1980.

Speaking in Rome on Monday, Vatican spokesman Federico Lom­bardi said that the pope made his decision to resign “aware of the great problems the church faces today,” adding that the decision showed “great courage and determination.” He insisted, however, that the pope’s decision was personal and that he had not resigned because of “difficulties in the papacy.” Benedict, Lombardi said, will not take part in the conclave to elect a new pope, adding that he is expected to retire to a monastery of cloistered nuns on the Vatican grounds.

Benedict emerged as a crusader and a lighting rod since the moment white smoke over the Sistine Chapel heralded his arrival in 2005. He encouraged a revival of the Latin Mass and promoted traditionalists in the Vatican hierarchy, determined to amplify the church’s message of morality and the role of Roman Catholicism as the one true faith. He sought to win back conservative Catholics opposed to the Second Vatican Council of 1962 and attempted to recruit new members, including Anglicans disenchanted with liberal views on female as well as openly gay clergy in their denomination. In 2006, he ignited street protests in the Islamic world after repeating a negative quote about the prophet Muhammad.

But his defenders have always said Benedict has been unfairly savaged by the media for actions that predated his tenure, and hailed his management of scandals, including issuing a rare official apology in 2010 to Catholics in Ireland for the widespread sexual abuse of children by clergy there in earlier decades. His decision to step aside to make way for a new and almost surely younger pope was hailed by many as another manifestation of his fierce generosity and goodwill.

The sudden end to his papacy presents the Vatican with a delicate choice: to elect a new pope who will represent continuity or one who will represent change. With the church declining in its former stronghold of Europe but finding its future in Latin America, Africa and Asia, pressure already was growing on the College of Cardinals — the global princes of the church — to break tradition by elevating a non-European pope.

Cardinal Donald Wuerl, archbishop of Washington, who last saw Benedict in Rome in October, said the announcement came as an “enormous surprise,’’ a statement that echoed sentiments worldwide.

“He presided at meeting after meeting after meeting,” Wuerl said. “There was no doubt that he was in full possession of his faculties.”

Benedict’s decision “says to me he is a very humble and honest person,” Wuerl added. “His love for the church is such that he has concluded it would be better not to try to lead this huge flock without the full strength of all of his energies.”

Some observers contend that the influence of the Vatican has receded under Benedict, in part because of his age but also because he operated in the shadow of his beloved and charismatic predecessor, John Paul II.

“He had a hard act to follow in John Paul, who was bigger than life,” said the Rev. Thomas Reese, a Catholic writer and former editor of America, a Catholic magazine. “Benedict suffered by comparison because he was much more shy, he wasn’t an actor, he preferred to write books and issue encyclicals rather than travel.”

Liberal Catholics have bemoaned his promotion of a generation of conservative bishops who believe that the church will hold together best if its teachings are communicated as black and white. A symbol to them has been the crackdown on the largest group of U.S. nuns, who the Vatican said were straying too far in their writings and lectures about homosexuality and contraception.

Traditional Catholics, however, celebrated his focus on orthodoxy.

“If you don’t sell full-throttle Catholicism, people are not going to buy it,” said George Weigel, who has written books about the church and the pope. “Everyone knows the whole package is more compelling and interesting than some sort of Catholic hors ­d’oeuvres that leave you hungry.”

Boorstein reported from Washington. Michael Birnbaum and Petra Krischok in Berlin, Eliza Mackintosh in London and James Arkin in Washington contributed to this report.