ROME (CNN) -- The spiritual leader of 1.2 billion Catholics, Pope Benedict XVI (16th), surprised the world Monday by saying 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.
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.
Benedict -- born Joseph Ratzinger -- will not be involved in choosing a new pope or in guiding the church after his resignation, Lombardi said.
Benedict was elected pope in 2005 after the death of Pope John Paul II, the third-longest-serving leader of the Catholic Church.
He has served during a time in which the church is declining in his native Europe but expanding in Africa and Latin America.
His papacy also has been marked with a series of scandals and controversies, including hundreds of new allegations of sexual abuse by priests.
Ratzinger was born 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.