He said Pope Benedict devoted his papacy to helping believers "encounter the core" of Catholicism through his writing on Christ.

Where Pope John Paul II was "such an extraordinary and dramatic figure" who had "a commanding public presence," Ruddy said, Pope Benedict's contribution will be "quiet and patient and enduring, almost as a seed grows into a plant."

Reese, of the Woodstock Theological Center, said modern medicine has made it more likely that future popes will outlive their ability to fulfill the responsibilities of leading the world's 1.2 billion Catholics.

While Vatican leaders made accommodations for Pope John Paul II, a former sportsman who was slowed in later years by Parkinson's disease and other ailments, canon law does not establish a process for removing an incapacitated pontiff.

Reese said he has "great respect and admiration" for Pope Benedict: "He put the Church first."

The last pontiff to resign was Gregory XII, who was pressured to step down in 1415 to help heal a dispute within the church known as the Great Western Schism. Celestine V, a former monk who cherished solitude, was the last to leave voluntarily, in 1294 after five months in the position.

For centuries, Catholics believed that to resign the papacy was to resist the will of God.

"There is a medieval aura around the job," said Michael Sean Winters, a columnist for the National Catholic Reporter. "The election is inspired by the Holy Spirit, and if you resigned you were betraying the Holy Spirit."

Ratzinger was an academic in his native Germany who served as a theological adviser to the archbishop of Cologne during the Second Vatican Council. He was elevated to archbishop of Munich and Freising in 1977, and named by Pope John Paul II to be prefect of the Congregation for the Doctrine of the Faith in 1981.

It was in that job as the Vatican's chief doctrinal enforcer that he drew international attention as a possible — some said likely — successor to Pope John Paul II.

He was elected pontiff in 2005. Early on, he angered Muslims when he quoted a 14th-century emperor who linked Islam and violence, and alienated Jews when the church reinstated the Latin Mass with its Good Friday prayer for their conversion.

But he later visited synagogues and a mosque, and met with Jewish and Muslim leaders during his trip to the United States. On each of the five days of that 2008 trip, he spoke of the crisis involving children who had been sexually abused by priests, and he met with abuse survivors in Boston, the epicenter of the scandal.

In his first encyclical as pope, he focused on love.

"He is a gentle, almost a courtly man," said Lori, the Baltimore archbishop, "and very, very gracious with a ready smile and a good sense of humor."

Lori called for prayers for the pontiff and for the Church.

"This is a solemn occasion for us, of course, but I hope we would recognize the humanity of this pope who recognized that the stamina required for this role wasn't his any longer," he said. "He readily announced this to the whole world. That takes a marvelously holy and strong person."

As the College of Cardinals meets at the Vatican, members won't be talking about ordaining women, expanding the married priesthood or celebrating same-sex marriage — those matters are settled in the eyes of church fathers.

Reese said the cardinals will ask who among them would be accepted worldwide and is familiar with sensitive matters in certain places, including the sex abuse scandal in America and other countries, and interfaith relations in countries such as India and Lebanon where Catholics and Muslims live side by side.

Martin, the editor at America, said the pope's successor will be a man of holiness and prayer who has the ability to lead a global organization and a zeal for spreading the Gospel

Martin called Pope Benedict's resignation a "noble and selfless act.

"Rare today is the person who would give up such immense power voluntarily," Martin said. "I was very touched by it."

Baltimore Sun librarian Paul McCardell contributed to this report.

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