In this file photo from 2000, Pope John Paul II greets World War II death camp survivor and boyhood friend Jerzy Kluger during a visit to Jerusalem. (Isaac Harari/AP)

Jerzy Kluger, a Polish Jew whose lifelong friendship with Pope John Paul II helped the pontiff’s efforts to repair Catholic-Jewish relations after centuries of anti-Semitism, died Dec. 31 at a hospital in Rome.

He was 90 and had Alzheimer’s disease, said his wife, Irene Kluger.

In the Polish town of Wadowice, where the pope and Mr. Kluger met as young boys in the 1920s, their friendship was a somewhat unusual one. Karol Wojtyla, who would become pope, came from a deeply Catholic family; Mr. Kluger came from an observant Jewish one.

For centuries, scholars have said, anti-Semitism was ingrained in Catholic teachings, which blamed Jews for the death of Christ.

During his papacy, from 1978 until his death in 2005, John Paul II was credited with doing more than any pope in history to unite the Catholic and Jewish faiths. His dedication to those efforts, church scholars said, stemmed in part from his intense friendship with Mr. Kluger, who lost much of his family in the Holocaust.

Mr. Kluger was a “link with the pope’s past,” said Eugene Fisher, the co-editor of “The Saint for Shalom: How Pope John Paul II Transformed Catholic-Jewish Relations.” “Perhaps the most profound change in Christianity was the change brought about in Catholic-Jewish relations,” he said.

Mr. Kluger and the pope knew each other by their boyhood nicknames, Jurek and Lolek. They met in grade school, played soccer in the streets and did homework together. Mr. Kluger, who became an engineer, let the young Karol see his math homework. The future pope let Jurek copy his Latin exercises, Mr. Kluger’s wife said.

One incident left a profound impact on Mr. Kluger. After learning that both boys had passed their high school exams, he ran to the church, where he knew he would find his friend, to share the news. Another parishioner recognized Mr. Kluger as a Jew and asked why he had come.

When Wojtyla heard about the exchange, he responded, “Aren’t we all God’s children?”

The young men lost touch during World War II, when Mr. Kluger was imprisoned in a Soviet labor camp before joining the Polish army and fighting with the Allies in Egypt and in Italy. After the war, with no family to return to in Poland, he moved to England and later to Rome, where he entered business.

The two men were reunited during the Second Vatican Council, which brought then-Archbishop Wojtyla to Rome. Mr. Kluger contacted his old friend, who immediately agreed to meet him.

The friendship was rekindled, and for the rest of their lives they remained in close touch. The newly installed Pope John Paul II surprised many Vatican-watchers by reserving his first private audience as pontiff for Mr. Kluger and his family.

Over lunches and dinners, the two men had intimate conversations that the pope enjoyed with few others, Fisher said. Mr. Kluger spoke openly, helping the pope understand Jewish sensibilities.

The New York Times reported that, while hospitalized after an assassination attempt in 1981, the pope asked Mr. Kluger to help him begin a diplomatic effort that in 1993 ended with the Vatican officially recognizing the state of Israel. Mr. Kluger was described as playing a behind-the-scenes but key role in the process.

John Paul II became the first pope to visit a synagogue and to visit the Auschwitz death camp. In 1998, he issued an official “act of repentance” for the Catholic Church’s failure to do more to stop the Holocaust. Critics contended that the apology did not go far enough. Mr. Kluger defended his friend.

“This pope is a friend of the Jewish people because he knows Jewish people,” Mr. Kluger told the New York Times in 1998. “He grew up in Wadowice.”

Jerzy Kluger was born April 4, 1921, in Krakow and grew up in Wadowice. His father was the president of the Jewish community and a successful lawyer. Mr. Kluger grew up “living like a prince,” his wife said, until the German invasion in 1939.

While fighting with the Polish army in Africa, Mr. Kluger met his future wife, Irene White, who was a driver for the British army. They were married in Egypt before Mr. Kluger fought at Monte Cassino, a key battle in the Italian campaign, in 1944.

After the war, Mr. Kluger received an engineering degree from the University of Nottingham and worked in that field before moving to Rome in the 1950s.

His daughter Leslie Kluger died in 2011.

Besides his wife, of Rome, survivors include his daughter Linda Kluger, also of Rome; a granddaughter; and two great-grandchildren. The children were described as having a close relationship with the pope, who permitted them to play with his skullcap.

In an interview with the New York Times, Mr. Kluger once explained his role in the pope’s outreach to the Jewish community.

“I was a friend,” he said. “We had friendly conversations, and friendly relationships which one way or another helped these developments. That’s all.”