John Cornwell has found damning evidence against saint-in-waiting Pope Pius XII. As those about to be canonised face increased academic scrutiny, becoming a saint has never been so tough.

Supplying the recent history of the papacy is no easy task because the Vatican archives maintain a 75-year rule of secrecy. Moreover, Eugenio Pacelli, Pope Pius XII - Hitler's pope as he has notoriously been called - was a secretive individual who, as far as we know, kept no diaries before he became pope and wrote few intimate letters.

That I have managed to tell Pacelli's story, the story of a man whose refusal to publicly denounce Hitler's murderous regime may have condemned millions of Jews to the gas chamber, is the result of unprecedented access to unpublished sources in closed archives in Rome - the collection of sworn depositions for the beatification of Pius XII in the keeping of the Society of Jesus.

Given that modern processes for beatification and canonisation are qualified by a strenuous search for evidence against the holiness of the "servant of God", these documents - 76 interviews conducted under oath 25 years ago - have proved crucial. Having consulted them, I now know how far Pacelli's guilt extended. By signing the Reich concordant in 1933, Pacelli assisted Hitler to power. In exchange for greater sway over the country's Catholics, the Catholic church in Germany withdrew from political action, weakening its ability to resist the terrors that followed.

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Arguments over Pacelli's reaction to the Final Solution have raged for more than 35 years, every attempt at a final verdict evoking a challenge from the opposite extreme (see box below). New generations are still attempting to come to terms with outstanding debts of conscience on the part of the papacy and the Catholic church 50 years after the end of the second world war. The evangelical churches of Germany acknowledged their guilt for the crimes of the Nazi regime, as did the Catholic hierarchy. The Holy See, by contrast, has made no such affirmation.

There have been papal initiatives to heal the breach between Christianity and Judaism: Paul VI's visit to Israel, John Paul II's two synagogue visits and his "remembrance" statement in spring 1998 on the history of offences against the Jews. But he used that last occasion to exonerate Pius XII's wartime conduct.

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While academic historians have toiled to unravel the truth, at the headquarters of the Jesuits in Borgo Santo Spirito a rather different investigation has been in progress. This is the research and writing of a positio, a "sacred" biography, in support of the beatification and, ultimately, the canonisation of Pacelli.

Beatification and canonisation are declarations by the pope that a dead individual has led a life of heroic virtue and resides in heaven.

Beatification indicates that the pope has sanctioned a local cult of the person's sainthood and that his person may be prayed to. Canonisation indicates the celebration of a worldwide cult. The positio is the story of an individual's holiness and must reflect the views of many people who knew him.

The process began in the autumn of 1964 when Pope Paul VI announced that the Congregation for Saints was to begin the canonisation procedure for Pius XII by an act of acclamation, bypassing the drawn-out process that can take centuries. The Jesuit order assumed responsibility for the process, and two specialist "saint-makers", Father Paul Molinari and Father Peter Gumpel, were appointed in 1965. Today, in their seventies, they are still at work.

Gumpel, a German of aristocratic origins whose family was persecuted by the Nazis, is the relator, the independent autonomous judge appointed by the pope to examine the materials presented in support of Pacelli. Over two years, as I worked in archives in Rome, I talked with Gumpel many times.

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The positio of Pacelli that Gumpel is overseeing has brought together hundreds of documents from many archives in Europe, but nobody outside the Congregation of Saints will have sight of it until a decision about the beatification has been made.

There is sure to be a highly controversial interim period leading up to beatification, if and when the pope makes Pacelli a "venerable" - meaning that he has sanctioned the penultimate stage of the process when the tribunal will scrutinise claimed miracles in support of the imminent declaration of Pacelli's "sainthood". Molinari and Gumpel both knew Pacelli personally and 40 years after his death are convinced of his sanctity.

Gumpel will not entertain the slightest criticism of Pacelli. This might indicate that his vast knowledge has brought him to an unassailably favourable conclusion, but my impression is that his information gathering has been somewhat selective. Comparing the rival works in the debate over Pacelli's wartime record, Gumpel praises Pinchas Lapide's The Last Three Popes and the Jews, while pouring scorn on the work of Robert Katz, Guenter Lewy and Saul Friedlander (summarised below), which he characterises as "unjustifiable and calumnious attacks".

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There have been criticisms of the beatification process in recent years because of the disappearance of the role of the "devil's advocate", an independent scrutiniser whose task it was to take account of criticisms of the servant of God. The new rules for the writing of the positio, dating from 1983, are meant to compensate for this loss by the incorporation of studies critical of the candidate.

But Gumpel's view of the matter, expressed in an essay in Catholic newspaper The Tablet, is that critics of Pacelli "should realise that they are trampling on the sensibilities of Catholics and in doing so hinder efforts to build better relations between the Catholic church and Jews".

This sort of special pleading only distances Gumpel from the role of academic historian. If better relations are to be built between the Catholic church and Jews, they will be achieved through Catholics heeding the pluralist narratives of history. I am convinced that history will show Pacelli to be, not a saintly exemplar for future generations, but a deeply flawed human being.

John Cornwell is a fellow of Jesus College, Cambridge, and author of Hitler's Pope (Viking), Pounds 20.00.

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