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 resigned 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," the pope said, according to the Vatican.

After Benedict's resignation becomes effective on February 28, cardinals will meet to choose a new leader for the church.

"Before Easter, we will have the new pope," the Rev. Federico Lombardi, a Vatican spokesman, said at a news conference.

The decision was not impulsive, he said.

"It's not a decision he has just improvised," Lombardi said. "It's a decision he has pondered over."

After his resignation, Benedict, 85, will probably retire to a monastery and devote himself to a life of reflection and prayer, he said. He will not be involved in choosing a new pope or in guiding the church after his resignation, Lombardi said.

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 recognize it to be a decision of great courage and characteristic clarity of mind and action," he said.

Archbishop Timothy Dolan, president of the U.S. Conference of Bishops, said he was 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. "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.

Benedict led the church as it saw declines in his native Europe but expansions in the developing world, including Africa and Latin America.

He also was known for his conservative views on theology and church doctrine. Dolan said he "warned of a dictatorship of relativism."

But his papacy, which began in 2005 after the death of Pope John Paul II, also was marked with controversy over the sexual abuse scandal in the Catholic Church.

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.

That same year, Benedict 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.

Abusive priests had "disfigured their ministry" and brought "profound shame and regret" on the church, he said at the time.

Barbara Blaine, president of the Survivors Network of those Abused by Priests, said Monday that 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 said.

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 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 when 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 John Allen Jr. 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 is the sixth German to serve as pope and the first since the 11th century.

The last pope to resign was Gregory XII in 1415. He did so to end a civil war within the church in which more than one man claimed to be pope.
Pope Benedict To Resign At The End Of The Month, Vatican Says