Pope Benedict XVI's surprise announcement Monday that he was stepping down as leader of the Roman Catholic Church may prove the signature event of his eight-year pontificate.

Benedict, a theological conservative and church traditionalist, took a step that a more moderate pope might not have dared: handing over the scepter of papal infallibility to someone yet to be chosen, something no pope had done in six centuries.

The 85-year-old Benedict has told confidants he will revert to his former status as a cardinal and reassume his birth name, Joseph Ratzinger. He decided to resign last March after a taxing visit to Mexico and Cuba, according to Monday's issue of L'Osservatore Romano, the Vatican newspaper.

A succession makes it likely that the next pope will visit Philadelphia in 2015, when the local archdiocese hosts the church's international World Day of the Family. Church officials expressed doubts last year that Benedict could make the long trip.

Although Benedict has indicated through his spokesman that he will not participate in the next conclave, he might signal his views on what the cardinals should look for in a new pope. If so, it would not be the first time.

On April 18, 2005, the opening day of the last papal conclave, Ratzinger delivered a stern homily at St. Peter's Basilica, warning his 114 fellow cardinals of a "dictatorship of relativism," and urging them to "resist the trends and novelties" sweeping the increasingly secularized West.

If it was a campaign speech, it worked. The next day, the cardinals elected the German-born theologian on the fourth ballot. Several who had come from far-flung archdioceses in Asia, Africa, and Latin America later admitted that Ratzinger was one of the few whose views they knew.

But the private and scholarly Ratzinger was probably less ambitious for the papacy than many of his red-cloaked colleagues, said the Rev. Thomas Reese, former editor of the Jesuit magazine America.

"I don't think he ever wanted to be on the world stage the way the papacy forced him to be," said Reese, author of several books on the Catholic hierarchy. "He was happiest with his books, and studying."

As many had predicted, Benedict did not change the church's long-standing positions barring married clergy, artificial birth control, homosexual acts, or the remarriage of divorced persons.

Although Benedict likely subscribes to the Catholic notion that the Holy Spirit guides the selection of a pope, his decision to step down was surely guided by his experience of watching his predecessor, John Paul II, decline over a half-decade due to Parkinson's disease.

In his final years, John Paul was obliged to rest for many hours a day, used a wheelchair, and became increasingly difficult to understand because of his slurred speech.

As John Paul's health deteriorated, Ratzinger and the rest of the Curia, or Vatican leadership, also watched as high-level decision-making normally reserved for a pope fell increasingly to John Paul's advisers, especially his confidant and secretary, Archbishop Stanislaw Dziwisz.

Those years spent watching his good friend decline, and a de facto pontiff move in to fill the vacuum, may have prompted Benedict to contemplate the need for a transfer of leadership in an era when medicine could prolong the life of an incapacitated pope.

That very topic was once so fraught that the Vatican would rarely acknowledge when a pope was ill. "The pope isn't sick until he's dead," went a long-standing joke.

But with Monday's announcement, Benedict appeared to put an end to that joke and the anxiety surrounding it. The famously traditionalist pontiff has created a new tradition - papal resignation - that promises to serve the Catholic Church well in the centuries ahead.

Contact David O'Reilly at 856-779-3841 or doreilly@phillynews.com or @doreillyinq on Twitter.