Ratzinger a close confidant of John Paul II
New pope oversaw church doctrine under his predecessor
Pope Benedict XVI greets the crowd in St. Peter's Square on Tuesday evening after he was elected pope.
Like his predecessor, Cardinal Joseph Ratzinger brings a long life story to his papacy.
The new pope receives mixed reaction around the world.
Why the new pope may have chosen Benedict as his papal name.
(CNN) -- The newly elected Pope Benedict XVI, Cardinal Joseph Ratzinger of Germany, was one of the most powerful men in the Vatican under Pope John Paul II, a strict enforcer of church doctrine who earned the nickname "Cardinal No."
As dean of the College of Cardinals, he delivered a stern homily Monday as he and his colleagues gathered to select a new pope, calling on them to be "true adults in the faith."
"Being an adult means having a faith which does not follow the waves of today's fashions or the latest novelties," he said. "A faith which is deeply rooted in friendship with Christ is adult and mature."
Since 1981, Ratzinger, who turned 78 on Saturday, has been in charge of the Congregation for the Doctrine of Faith, the Vatican office that oversees "the doctrine on the faith and morals throughout the Catholic world," according to the Vatican.
Ratzinger became known as "Cardinal No" because of his drives to crack down on the liberation theology movement, religious pluralism, challenges to traditional teachings on issues such as homosexuality, and calls to ordain women as priests.
Liberation theology combined Christian theology with political activism on issues like human rights and social justice. While partially compatible with Catholic social teachings, it was rejected by the Vatican, which objected to the mixing of church theology with Marxist ideas like class struggle.
"It has been his job to police the doctrinal boundaries of the Roman Catholic Church, and inevitably when you do that there are going to be hurt feelings by people who find themselves on the wrong side of those lines," said CNN Vatican analyst John Allen Jr., a correspondent for the National Catholic Reporter and author of "Cardinal Ratzinger: The Vatican's Enforcer of the Faith."
'A dictatorship of relativism'
Ratzinger has said modernity led to a blurring of sexual identity, causing some feminists to become adversaries of men. He labeled homosexuality "an intrinsic moral evil."
He argued that Muslim Turkey did not belong in Christian Europe and issued a document saying that Catholicism was the only true religion, questioning the validity of other religions, even Christian ones, even as his Pope John Paul II was trying to reach out to other faiths.
Although objections came even from some of his fellow cardinals, the pope did not restrain Ratzinger, in part because their friendship went back four decades, to the time when the two were young priests at the Vatican II meetings in Rome.
In 1984, Ratzinger caused a major diplomatic stir with a condemnation of communism, calling the Soviet Union and its satellites "a shame of our time."
After the collapse of the communist regimes, he also condemned the excesses of capitalism and cautioned, "We must coordinate the free market with the sense of responsibility of one towards the other."
He tangled with other cardinals and disciplined church officials who dissented from official church policy.
He rejected a 1993 pastoral letter co-written by fellow German Cardinal Walter Kasper that encouraged divorced and civilly remarried Catholics to return to the sacraments.
After Kasper came to Rome in 1999 to take over the Vatican's ecumenical affairs office, he and Ratzinger jousted in a series of journal articles.
In them, Ratzinger argued for centralized authority over the church and Kasper advocated the equality of the local church with the universal church.
In his homily at Monday's eligendo Summo Pontifice Mass before the cardinals began the conclave, Ratzinger warned against "relativism" and said having a strong faith, based on church teaching, is often labeled today as a "fundamentalism."
"We are moving toward a dictatorship of relativism which does not recognize anything as for certain and which has as its highest goal one's own ego and one's own desires," he warned.
Allen said Ratzinger's homily expressed his core message that the church must remain true to itself.
"If you want to boil it down, it is the kind of modern idea that there is no truth with a capital 'T.' That is, that absolute truth does not exist; that you have your truth, I have mine, and we sort of go our separate ways," Allen said.
"Ratzinger, in concert with a very strong current in church tradition, is insisting that is not true. That, in fact, there is absolute truth, and the pinnacle expression of that truth came in the person of Jesus Christ.
"And therefore, that the church must have the courage to proclaim that absolute truth, even in an era that doesn't want to hear it."
Son of a police officer
Ratzinger was born on April 16, 1927, in Marktl Am Inn, Germany. He was the son of a police officer who came from a traditional family of farmers in Lower Bavaria, according to his Vatican biography. Bavaria remains a heavily Catholic region of Germany.
He spent his adolescent years in Traunstein, near the Austrian border, when the Nazi regime of Adolf Hitler controlled Germany.
In his memoirs, Ratzinger wrote that school officials enrolled him in the Hitler Youth movement against his will when he was 14 in 1941.
Membership was compulsory and the officials enrolled his entire class, acting on orders from the Nazi regime, Allen said. Ratzinger said he was soon let out because of his studies for the priesthood.
According to Allen, his family was quietly strongly anti-Nazi, and his father took a series of less significant jobs to stay away from what was happening in Nazi Germany.
During World War II, Ratzinger was drafted into army in 1943, serving in an anti-aircraft unit that tracked Allied bombing raids.
He deserted in the waning months of the war in 1945 and returned to Traunstein, where he was taken prisoner by U.S. troops.
In June 1945, he was released from a POW camp and returned home, this time hitching a ride on a milk truck.
From 1946 to 1951, he studied philosophy and theology at the University of Munich and at another school in Freising.
He was ordained a priest in 1951. In 1953, he received his doctorate in theology. His doctoral thesis was entitled, "The People and House of God in St. Augustine's doctrine of the Church."
Four years later, he was qualified as a university teacher and taught dogma and fundamental theology at four different German universities.
In 1962, at age 35, he was a consultant during Vatican II to Cardinal Frings, a reformer who was the archbishop of Cologne, Germany.
Allen said that as a young priest Ratzinger was on the progressive side of theological debates, but began to shift right after the student revolutions of 1968.
In 1969, he was named professor of dogmatic theology and of the history of dogma at the University of Regensburg, where he was also named vice president.
'A simple, humble worker'
Allen writes that Ratzinger is a shy and gentle person whose former students speak of him as one of the best-prepared and most caring professors they every encountered.
In March 1977, he was named archbishop of Munich and Freising by Pope Paul VI. When he was consecrated on May 28, 1977, he was the first diocesan priest after 80 years to take over the pastoral ministry of the large Bavarian diocese.
The next month, he was elevated to cardinal by Pope Paul VI on June 27, 1977. He was archbishop of Munich until November 25, 1981, when he was nominated by Pope 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.
Ratzinger is highly regarded as an intellectual and considered humble and charming personally.
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 routinely held Mass for German students and seminarians on Thursdays at the Vatican, said Brother Sylvester Herman, a German priest who was in St. Peter's Square when the new pope took to the balcony.
"He's a very nice guy. I was always impressed with the simplicity with which he talks to you," Herman said. "He would always have time and be very attentive and listen."
As pope, Allen said Ratzinger will have to be a "universal pastor."
"He is usually presented in these very stern, inquisitorial visages because that aligns with the public perception of the man. But that's not the man we saw tonight, and I think it is going to be interesting to see how he carries that forward," Allen said.
He is the sixth German to serve as pope and the first since the 11th century. He will lead the church after the third-longest papacy in church history and during a time in which the church is declining in his native Europe but expanding in Africa and Latin America.
"In a very difficult, complex time where there are challenges of traditional moral values that are very commonplace in our world, he really has been a kind of a rock of Gibraltar, so to speak -- a really solid foundation as we look to the future," said Bishop William Skylstad, president of the U.S. Conference of Catholic Bishops.
"As we look to the future, I think he will continue to be that as the Holy Father."
CNN's Flavia Taggiasco contributed to this report.