The spiritual leader of 1.2 billion Roman Catholics, Pope Benedict XVI, surprised the world Monday by saying he will resign at the end of the month "because of advanced age."

It's the first time a pope has stepped down in nearly 600 years.

"Strength of mind and body are necessary, strength which in 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," said Benedict, 85, according to the Vatican.

The news startled the Catholic world and led to frenzied speculation about who would replace him, including a debate about the merits of naming a pontiff from the developing world, where the church continues to grow, versus one from Europe, where it has deep historical roots.

But that decision will not be made by Benedict, who will leave his post at 8 p.m. on February 28, said the Rev. Federico Lombardi, a Vatican spokesman.

"Before Easter, we will have the new pope," he said.

While Benedict won't be directly involved in his successor's selection, his influence will undoubtedly be felt. He appointed 67 of the 117 cardinals that -- as of Monday -- are set to make the decision.

The number of electors could drop to 115, as two cardinals will turn 80 in March, when their age makes them ineligible to cast a vote. More than two thirds of whatever the final number must agree on the next pope, a decision that will be announced to the world in the form a puff of white smoke emerging from a chimney in the Vatican.

CNN Senior Vatican Analyst John Allen said that means the next pope, no matter where he is from, will probably continue in Benedict's conservative tradition, which has seen the church take a firm line on issues such as abortion, birth control and divorce.

Resignation 'not a decision he has just improvised'

The pope, born Joseph Ratzinger, will first head to the pope's summer residence before he likely retires to a monastery and devotes himself to a life of reflection and prayer, Lombardi said. He won't be involved in managing the church after his resignation.

In a sign of just how rare an event this is, church officials aren't sure what the pope will be called after he leaves the office.

One possibility, Allen said, is "bishop emeritus of Rome."

While not quite unprecedented, his resignation is certainly historic. The last pope to step down before his death was Gregory XII, who in 1415 quit to end a civil war within the church in which more than one man claimed to be pope.

Benedict took months to decide that he wasn't up for the job anymore, Lombardi said.

"It's not a decision he has just improvised," the Vatican spokesman said.

A family friend in Regensburg, Germany, told CNN that Benedict had been thinking resigning for some time because of his age. He had discussed the decision with his older brother, the Rev. Georg Ratzinger, according to the friend, who asked not to be named because he does not speak for Georg Ratzinger.

Several years ago, Benedict had suggested he would be open to resigning should his health fail, Allen said. But no one expected him to do so this soon.

Jannet Walsh, a CNN iReporter from Murdock, Minnesota, counts herself among those "shocked" by the news of his resignation. But she said it was an admirable decision because he seemingly recognized that he could not adequately "carry out his office as pope" given his health.

"The resignation of the pope is actually a very selfless act, and it should be applauded," Walsh said.

Benedict's legacy

Joseph Ratzinger was born and raised in Germany, where he briefly served in the Hitler Youth, despite his objections, and the German military during World War II. He then established himself as a leading theologian, professor and local Catholic leader before rising up the Vatican ranks.

He was dean of the College of Cardinals in 2005 when he became the sixth German to be picked as pope, albeit the first since the 11th century. At the time, the church was facing several pressing issues, including declining popularity in parts of the world and a growing crisis over the church's role in handling molestation accusations against priests around the world.

Given his age at the time -- 78 -- he was widely seen as a caretaker pope, a bridge to the next generation after the long tenure of John Paul II, a popular, globe-trotting pontiff whose early youth and vigor gave way to such frailty in later years that he required help walking and was often hard to hear during public addresses.

As an aide to John Paul, Benedict served as a strict enforcer of his conservative social doctrine. To no one's surprise, he continued to espouse a conservative doctrine after taking the office himself. He frequently warned of a "dictatorship of relativism."