Held hostage by the Vatican

In one of the most eloquent inter-faith gestures of his pontificate, John Paul II became in 1986 the first pope ever to visit a synagogue. He joined Rome's Jewish community at their place of worship on the Lungotevere and apologised with heartfelt sincerity for Christianity's long history of anti-Semitism.

The gruesome details of the Church's almost 2,000 years of persecution of its spiritual older brothers and sisters are well known - from the medieval demonising of the Jews by the Inquisition, through successive expulsions and massacres, down to the Vatican's shameful silence in the face of the Holocaust. The idea that Jews had committed deicide by killing Jesus was only finally refuted by Rome in 1965. Until then it had fuelled one of the most hateful vendettas in the history of humankind.

John Paul's gesture of reconciliation had a much more particular - if widely neglected - context, as David Kertzer now reveals. Just over a century earlier, Pope Pius IX had shown his utmost contempt for Rome's Jewish community when he refused their anguished pleas for the return of six-year-old Edgardo Mortara to his Jewish parents. Where John Paul sat with head bowed, listening to the synagogue choir singing a hymn chanted by Jews on their way to the Auschwitz gas chambers, another pope at the dawn of the modern period denied Jews any civil rights and played God over the future of their children.

Edgardo was seized in 1858 at dead of night from his loving parents by the Inquisition. The boy, it was said, had been secretly baptised by a Catholic servant girl in the Mortara household when she feared for his life during an illness. According to the perverse logic of the time, that made him a Catholic - and no Catholic could be brought up by Jewish parents.

After they had got over their initial shock, the Mortaras acted like many other victims of miscarriages of justice since. They kept believing someone in authority would realise that it was all a terrible mistake. While they patiently petitioned Pope Pius to overturn the Inquisition's decision, word of their ordeal spread first through Italy's Jewish communities, then to Jews in Europe and North America, and finally to national and world political leaders.

Kertzer, an American social anthropologist, follows the twists and turns of this cause celebre. On one side was the most reactionary and fanatical of popes, stuck in a medieval time-warp. On the other, a new wave of hardened anti-clerics and opportunistic politicians such as Count Cavour, who exploited the fate of Edgardo in order to precipitate Italian reunification.

Kertzer argues persuasively that Pius's handling of the whole melodrama provided Cavour with all the ammunition he needed to convince the French Emperor Napoleon III to move against the remaining papal territories. Pius, in short, demonstrated why his temporal powers were a dangerous anachronism.

Caught in the middle were the Mortara parents and their relatives, mentally and physically destroyed by separation from their son. Out of this human drama, Kertzer fashions an illuminating history of the decline of church power and the rise of an Italian nation.

Taking the lives of "ordinary" people in extraordinary circumstances to shed new light on the past is a tricky undertaking. All too often the attempt ends up lost between narrative and analysis. Kertzer, however, has produced a triumph that deserves to stand alongside such classics as Natalie Davies's The Return of Martin Guerre and Aldous Huxley's The Devils of Loudon.

The campaign to free Edgardo Mortara had mixed results. Papal power crumbled, as it would surely have done anyway. Italy was reunited. The links between Jews around the globe were immeasurably strengthened. For the first time they co-ordinated a campaign against an oppressor and persuaded public opinion and politicians to back them.

But for the Mortaras there was no happy ending. On the fall of Rome the teenage Edgardo fled to Austria rather than face his parents. He was soon afterwards ordained a priest and spent the rest of his life as a travelling evangelist - a strange vaudeville turn, reworking his own story to convince others to convert. He never saw his father again, and only late in life re-established contact with his mother and siblings. He died in 1940 in Belgium, just as the Nazis began rounding up Jews for the concentration camps.