At the beginning of the second World War, more than 80 per cent of all the Jews in the world were either living in Poland or descended from people who had lived within historic Polish boundaries. Despite that (or perhaps because of it) Poland was a place where some of the most virulent forms of anti-Semitism festered, and often enough erupted in open violence. Much of this anti-Semitism was Christian - and, specifically, Catholic - in origin.
In 1936, the Catholic primate of Poland, in a pastoral letter, urged a boycott of Jewish businesses, describing the Jews as people who "deceive, levy interest, and are pimps". Poland's largest Catholic publishing house, managed by the Franciscans, published an influential newspaper which portrayed Jews as interlopers who could never become true Poles if they did not convert to Catholicism. Other newspapers, in text and cartoons, made the Franciscans' modest efforts seem positively well-mannered.
In Wadowice, a county town close to Krakow, Jews were a minority. They had been forbidden even to live in the town until 1868, but by the 1930s their influence had grown. They amounted to 20 per cent of the population, and owned 40 per cent of the shops. There were tensions, but they seemed for the most part to be manageable. A Jewish businessman accused of sacking a Catholic worker for religious reasons was acquitted - by Catholic judges. Here and there, common humanity broke through.
One of the places where Jews and Christians inevitably mixed was the local high school, where two students struck up a friendship which was to have extraordinary consequences. One of them was Jurek, or Jerzy, Kluger, the son of a wealthy Jewish lawyer. The other was Karol Wojtyla, son of a retired army officer. They played cowboys and Indians together; the preferred role was that of the Indian underdog, understandable in that Poland had won its independence only 10 years earlier.
In many ways the two friends were temperamentally and psychologically distinct. Kluger was bright, mischievous, athletic and impulsive. Wojtyla was reflective, literary and quiet. But there were two areas in which their talents blended: one was drama, the other football.
Wojtyla became, in his own words, "completely absorbed by a passion for literature, especially dramatic literature, and for the theatre". While at school he directed and played the lead in patriotic plays, embodying themes that Polish Christians and Jews could share. Later, after the outbreak of the war, he was, as a participant in a clandestine university, to write verse dramas based on biblical themes about the plight of all Poles under the Nazis.
The ancient divisions between Christian and Jew, however, tended to manifest themselves in one school activity among all others - on the football pitch. Despite the often justified apprehension of their teachers, the boys organised their teams along religious lines. But in Wadowice there weren't always enough Jews to make up a team. A particular crisis would occur when their big goalie, Polek Goldberger, couldn't play. And so it happened that the future Pope John Paul II, not least because of his friendship with the son of the wealthy Jewish lawyer, ended up defending the Jewish goal against his co-religionists.
The two friends - friendship seems an almost inadequate word to describe a relationship that was both intense and joyful - were separated by the war. Wojtyla remained to become a priest in Poland; Kluger and his father were deported to the Soviet Union. But that image of the strapping young student playing in goal for the Jews was, in a way, prophetic.
The scene shifts to November 1965. Jerzy Kluger is now a businessman living in Rome, and has suppressed most of his childhood memories, and lost touch with all his childhood friends. He is driving towards the city from the south, when his companion in the car reads out loud a small newspaper item about a recent speech by a Polish bishop urging the church to prepare itself for the modern world. As the bishop's name is read out, Kluger goes into a reverie. When he gets back to the city, he makes a few phone calls: within hours, he and his friend Lolek are reunited.
This might be the end of the story, but in a sense it is only a beginning. The rediscovered friendship puts down fresh roots - this time institutional and political as well as personal and emotional. Kluger - whose role in all of this remains to all intents and purposes unknown - becomes a vital middle-man in the rapprochement which is taking place between Christians and Jews and - more specifically, between the Vatican and the State of Israel.
The late Darcy O'Brien, in an astonishing new book, has now given us a fascinating micro-history of the role this friendship has played in what has undoubtedly been one of the most important initiatives of the present papacy. Along the way, he gives us extraordinary insights into Poland itself, into the inner workings of the Vatican (there is a marvellous pen-picture of two cardinals and two Jews playing bridge together within sight of the dome of St Peter's) and, more especially, into the complex personality of the present Pope.
When Pope John Paul II was described as a "Polish liberal", O'Brien observes, few people realised that the adjective governed, rather than merely qualified, the noun. And it is difficult to imagine any future book being written about John Paul II that will not draw substantially on this detailed portrait of a man whose complexities and whose cultural background are fully weighed and analysed, sympathetically and with style.
It is clear that Kluger played a key role in all of this. So, evidently, did Cardinal Deskur, whose titular job as head of the Vatican's Commission on Communications evidently left him plenty of time for politicking at a senior level in this most delicate of areas. The delicacy can be exemplified in two of the crunch moments: the damage limitation exercise taken after John Paul II had met Arafat; and the extraordinary - it seems even now - papal silence after the massacres at Sabrah and Shatila. Against that, there is an eyewitness account of the Pope's speech at Auschwitz, and an analysis of some of his other speeches on similar topics, which adds an important dimension to the often superficial study of his policies.
Individual incidents apart, O'Brien leaves us in little doubt that the Pope's initiatives in Christian-Jewish relations are born, not just of the geo-political agenda to which pontiffs and other world leaders are drawn as moths to a flame, but by something deeper - something personal, religious, and an important way quintessentially Polish as well. His story is in itself a labour of love, and well worth the telling.
The Hidden Pope by Darcy O'Brien, HarperCollins, £17.99 in UK