BERLIN — The man chosen as pope yesterday grew up in the foothills of southern Germany during the rise of Nazism and was an architect of theological reform before abandoning those ideas for a rigid conservatism to battle what he saw as threats from secularism and leftist politics.

The son of a Bavarian police officer, Joseph Ratzinger, 78, grew up in the cauldron of World War II, like John Paul II, and became a man as the Cold War reached across Europe.

Born April 16, 1927, in the town of Marktl am Inn, Ratzinger spent his adolescent years in the Bavarian city of Traunstein. His family opposed the rise of the Nazi party in the 1930s. Like most German teenagers in the early 1940s, he became a member of a Hitler Youth group. Interrupting his seminary studies at the age of 17, he was assigned to assist an anti-aircraft unit.

GETTY IMAGES Pope Benedict XVI as a German air-force aide in this 1943 photo.

"The horrors of the Reich were right there in Traunstein, staring Ratzinger in the face, just outside the door of the gymnasium or across the seminary playing field," wrote John L. Allen Jr. in his biography, "Cardinal Ratzinger: The Vatican's Enforcer of Faith." Many of the region's Jewish citizens were either sent to Dachau concentration camp or fled Germany.

In a recent interview with German media, Ratzinger's brother, Georg, also a priest, said it was impossible to resist Nazism. But Allen said the would-be pontiff viewed the church as a buttress against Nazi evils, adding that Ratzinger's father and the local pastor criticized the Reich.

Rabbi Marvin Hier of the Simon Wiesenthal Center in Los Angeles agreed that Ratzinger's father was an anti-Nazi and said Ratzinger's membership in the Hitler Youth should not be taken as an indication of Nazi sympathies because membership was mandatory. Hier said he expects him to continue John Paul II's outreach to Jews.

Ratzinger was released from the anti-aircraft unit in September 1944, according to Allen's account, and was drafted into the German army. He deserted in April or May 1945 and was briefly held as a prisoner of war by U.S. forces near his home in Traunstein.

He studied at St. Michael's Seminary in Traunstein, and he and his brother were ordained priests on the same day in 1951.

Ratzinger went on to study philosophy at the University of Munich and received a doctorate in theology. An accomplished pianist who enjoys taking long walks in the German mountains, Ratzinger was groomed early as a Catholic intellect.

Even critics call him urbane and cultured, and according to the Vatican, he speaks eight languages including German, English, French, Italian, Portuguese and Spanish.

He was a progressive and eloquent voice during the 1962-65 Second Vatican Council, when the church became more open under Pope John XXIII. He helped draft an attack on the church laws dealing with heresy, which dated to medieval times and which the draft said were a "source of scandal" to the world.

Ratzinger's reform-minded tendencies, however, began shifting in the late 1960s. As a professor at Tuebingen University, he opposed Marxist student demonstrations and left the institution for another teaching post closer to his home in Bavaria.

In a 1997 interview with the publication "Salt of the Earth," Ratzinger described such political ideologies as "tyrannical, brutal and cruel. That experience made it clear to me that the abuse of the faith had to be resisted."

In 1972, Ratzinger and other theologians started a Catholic journal, "Communio." Five years later, he was named Archbishop of Munich and was elevated to cardinal by Pope Paul VI.

Pope John Paul II assumed the papacy in 1978, and Ratzinger's intellect and doctrinal writings impressed the new pontiff. In 1980, he appointed Ratzinger to chair a Synod on Laity.

John Paul believed he had found a kindred voice in Ratzinger, who agreed to the pope's 1981 request to head the Vatican's Congregation for the Doctrine of Faith, a position he held until this month.

Vibrant and strong in his beliefs, Ratzinger is also known as a quiet, almost shy man. Friends and critics alike describe him as an engaging man who can talk on topics ranging from classical music to the Gospels.

"Cardinal Ratzinger is known for his gentleness and timidity," said Mario Marazziti, a leader of the Saint Egidio Community, a Catholic movement that works with the poor. "When people greeted him crossing St. Peter's Square, he seemed almost stunned that people recognized him."

The Rev. Caesar Atuire, who organizes pilgrimages for the Vatican, said: "Before you meet him you hear he is one of the 'watchdogs' of faith. And then you meet a simple guy, with almost a simple smile on his face, as if he's scared to hurt anybody."

But his doctrinal fervor has gained him many detractors and set him against former colleagues, including the liberal German theologian Hans Kung, who was influential in getting Ratzinger hired at Tuebingen University. Ratzinger criticized Kung's views, and in 1979 the Vatican suspended his license to teach theology.

About one-third of Germany's population of 82 million is Catholic. Like in other European nations, church attendance has dwindled over the past decade and Ratzinger's stands against abortion counseling, homosexuality and ecumenical communion services have angered many in this nation, which instigated the Protestant Reformation against the Catholic Church in the 16th century.

"He's insulted other religions, and it's very disappointing," said Christian Weisner, a leading member of the international We Are the Church movement that seeks a more open church.

Weisner and other critics added that the Vatican's 2000 document "Dominus Jesus" — which was written chiefly by Ratzinger and asserted the primacy of Catholicism while branding other religious groups deficient — was a setback in relations with Muslims and other denominations.

Geraldine Baum in Rome, Teresa Watanabe in Los Angeles and Christian Retzlaff and Petra Falkenberg in Berlin contributed to this report.