Rome (CNN) -- The spiritual leader of 1.2 billion 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 and shocked the Catholic world and led to frenzied speculation about who would replace him.
Analysts and experts immediately began debating 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.
Cardinals will meet to choose Benedict's successor sometime after his official resignation on February 28, the Rev. Federico Lombardi, a Vatican spokesman, said at a news conference.
"Before Easter, we will have the new pope," he said.
Benedict won't be involved in the decision, Lombardi said. But his influence will undoubtedly be felt. Benedict appointed 67 the 118 cardinals who will make the decision.
Photos: Pope Benedict XVI Photos: Pope Benedict XVI
CNN Senior Vatican Analyst John Allen said that means the next pope, no matter where he is from, will likely continue in Benedict's conservative tradition -- which has seen the church take a firm line on issues such as abortion, birth control and divorce.
The pope, born Joseph Ratzinger, is likely to retire to a monastery and devote 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."
enedict will become the first pope to resign since Gregory XII in 1415. In that case, Gregory quit to end a civil war within the church in which more than one man claimed to be pope.
In this case, it wasn't external forces but the ravages of time that forced Benedict's hand. After months of consideration, he concluded he just wasn't up to the job anymore, Lombardi said.
"It's not a decision he has just improvised," Lombardi said. "It's a decision he has pondered over."
Benedict has been thinking about resigning for some time because of his age, a family friend in Regensburg, Germany, told CNN Monday. He has discussed the resignation 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, he said.
According to Lombardi, Benedict will step down as pope at 8 p.m. February 28 in Rome, then head for the pope's summer residence. He will probably move to a monastery in the Vatican after that, Lombardi said.
After the resignation takes effect, cardinals will gather in Rome to select a successor. It takes at least two-thirds plus one of the 118 voting cardinals to elect a new leader for the church.
Benedict took over as pope in 2005 as the church was facing a number of 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 following the long reign 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 assistance 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."
"In a world which he considered relativist and secular and so on, his main thrust was to re-establish a sense of Catholic identity for Catholics themselves," said Delia Gallagher, contributing editor for Inside the Vatican magazine.
Where John Paul wowed crowds around the world with his mastery of numerous languages, Benedict took his training as a college professor to the Vatican and will be seen at his most influential in years to come with his writings, Gallagher said.
Allen called Benedict a "great teaching pope."
Benedict also worked to advance religious freedom and reduce friction among adherents of various faiths, said Bill Donohue of the U.S. Catholic League.
"The pope made it clear that religious freedom was not only a God-given right, it was 'the path to peace,'" Donahue said.
Sex abuse scandal
Benedict became pope at the height of the molestation scandal involving Catholic priests, with complaints of sexual abuse and lawsuits over the issue tearing at the church.
Abusive priests had "disfigured their ministry" and brought "profound shame and regret" on the church, Benedict said in 2010, the same year he issued new rules aimed at stopping abuse.
The rules included allowing church prosecution of suspected molesters for 20 years after the incidents occurred, up from 10 years previously. The rules also made it a church crime to download child pornography and allowed the pope to remove a priest without a formal Vatican trial.
"No one did more to successfully address the problem of priestly sexual abuse than Joseph Ratzinger," Donohue said.
While he received praise for the changes, some also accused him of doing too little to stem the abuse, both while pope and before he took office.
In 2010, The New York Times reported that church officials, including Ratzinger, had failed to act in the case of a Wisconsin priest accused of molesting up to 200 boys. The Times reported that church officials stopped proceedings against the priest after he wrote Ratzinger, who was at the time the cardinal in charge of the group that oversees Catholic Church doctrine.
Ratzinger never answered the letter, according to the Times, and church officials have said he had no knowledge of the situation. But a lawyer who obtained internal church paperwork said at the time that it "shows a direct line from the victims through the bishops and directly to the man who is now pope."
Also in 2010, the Times reported that the future pope -- while serving as the archbishop in Munich -- had been copied on a memo informing him that a priest accused of molesting children was being returned to pastoral work. At the time, a spokesman for the archdiocese said Ratzinger received hundreds of memos a year and it was highly unlikely that he had read it.
Barbara Blaine, president of the Survivors Network of those Abused by Priests, issued a statement Monday saying the church should choose a new pope dedicated to preventing sexual abuse by priests.
"For the Church to truly embody the spiritual teachings of Jesus Christ, it must be led by a pontiff who demands transparency, exposes child-molesting clerics, punishes wrongdoers and enablers, cooperates with law enforcement, and makes true amends to those who were hurt so greatly by Catholic priests, employees and volunteers," Blaine wrote.
Victims' groups are pressing the International Criminal Court to prosecute Benedict in the sex abuse scandal, and say the resignation won't change that, according to Pam Spees, of the public policy law firm Center for Constitutional Rights, which is helping SNAP pursue the case.
Benedict's decision was not expected by leaders and everyday Catholics around the world.
Archbishop Vincent Nichols, the president of the Catholic Bishops' Conference of England and Wales, said the decision "shocked and surprised everyone."
"Yet, on reflection, I am sure that many will recognise it to be a decision of great courage and characteristic clarity of mind and action," he said in a written statement.
Archbishop Timothy Dolan, president of the U.S. Conference of Bishops, said he was startled, and sad, to see Benedict resign.
"The Holy Father brought the tender heart of a pastor, the incisive mind of a scholar and the confidence of a soul united with His God in all he did," he said in a written statement. "His resignation is but another sign of his great care for the Church."
British Prime Minister David Cameron said Benedict "will be missed as a spiritual leader to millions." Cameron's Irish counterpart, Enda Kenny, praised Benedict for decades of leadership and service, as well as his decision to resign.
"It reflects his profound sense of duty to the Church, and also his deep appreciation of the unique pressures of spiritual leadership in the modern world," Kenny said in a prepared statement.
Life before the papacy
Benedict was born Joseph Ratzinger on April 16, 1927, in Marktl Am Inn, Bavaria, a heavily Catholic region of Germany.
He spent his adolescent years in Traunstein, near the Austrian border, during the Nazi regime of Adolf Hitler.
Ratzinger wrote in his memoirs that school officials enrolled him in the Hitler Youth movement against his will in 1941, when he was 14.
He said he was allowed to leave the organization because he was studying for the priesthood, but was drafted into the army in 1943. He served with an anti-aircraft unit until he deserted in the waning days of WW II.
After the war, he resumed his theological studies and was ordained in 1951. He received his doctorate in theology two years later and taught dogma and theology at German universities for several years.
In 1962, he served as a consultant during the pivotal Vatican II council to Cardinal Frings, a reformer who was the archbishop of Cologne, Germany.
As a young priest, Ratzinger was on the progressive side of theological debates, but began to shift right after the student revolutions of 1968, CNN Vatican analyst Allen said.
In his book "Cardinal Ratzinger: The Vatican's Enforcer of the Faith," Allen says Ratzinger is a shy and gentle person whose former students spoke of him as a well-prepared and caring professor.
Pope Paul VI named him archbishop of Munich in 1977 and promoted him to cardinal the next month. Ratzinger served as archbishop of Munich until 1981, when he was nominated by John Paul II to be the head of the Congregation for the Doctrine of the Faith, a position he held until his election as pope.
He became dean of the College of Cardinals in November 2002 and in that role called the cardinals to Rome for the conclave that elected him the 265th pope.
In his initial appearance as pope, he told the crowd in St. Peter's Square that he would serve as "a simple and humble worker in the vineyards of the Lord."
He was the sixth German to serve as pope, but the first since the 11th century.