Historians can now pore over secret files from the papacy of Pius XII, who has long faced accusations of being a Nazi sympathiser

New light will be shed on one of the most controversial periods of Vatican history on Monday when the archives on Pope Pius XII – accused by critics of being a Nazi sympathiser – are unsealed.

A year after Pope Francis announced the move, saying “the church isn’t afraid of history”, the documents from Pius XII’s papacy, which began in 1939 on the brink of the second world war and ended in 1958, will be opened, initially to a small number of scholars.

Critics of Pius XII have accused him of remaining silent during the Holocaust, never publicly condemning the persecution and genocide of Jews and others. His defenders say that he quietly encouraged convents and other Catholic institutions to hide thousands of Jews, and that public criticism of the Nazis would have risked the lives of priests and nuns.

“The opening of the archives is decisive for the contemporary history of the church and the world,” said Cardinal José Tolentino Calaça de Mendonça, the Vatican’s archivist and librarian last week.

Bishop Sergio Pagano, the prefect of the Vatican Apostolic Archive, said scholars would have to make a “historical judgment”. He added: “The good [that Pius did] was so great that it will dwarf the few shadows.” Evaluating the millions of pages in the archives would take several years, he said.

More than 150 people have applied to access the archives, although only 60 can be accommodated in the offices at one time. Among the first to view the documents will be representatives of the Jewish community in Rome, and scholars from Yad Vashem, Israel’s Holocaust museum, and the United States Holocaust Memorial Museum.

David Kertzer, an American expert on the relationship between the Catholic church and fascism, who will begin examining the papers this week, said there were “signs of nervousness” at the Vatican about what would emerge from the archives. The Vatican archives would provide an “immense amount of fresh material from many millions of pages”, he told the Observer.

“On the big question, it’s clear: Pius XII never publicly criticised the Nazis for the mass murder they were committing of the Jews of Europe – and he knew from the very beginning that mass murder was taking place. Various clerics and others were pressing him to speak out, and he declined to do so.

“Although there is a lot of testimony showing that the church did protect Jews in Rome, when more than 1,000 were rounded up on 16 October 1943 and held for two days adjacent to the Vatican [before deportation to the death camps], Pius decided not to publicly protest or even privately send a plea to Hitler not to send them to their deaths in Auschwitz. Hopefully, what we’ll find from these archives is why he did what he did, and what discussions were going on behind the walls of the Vatican.”

Mary Vincent, professor of modern European history at Sheffield University, said that much of the criticism of Pius Xll lacked nuance. “He was a careful, austere and quite unlikable man, trying to steer a path through almost impossible circumstances. He had clear views about what he saw as the threat of Soviet communism, and his view of Italian fascism was quite a bit softer. But categorising him as good or bad is not helpful – it’s about the decisions he took, and the space he had to make those decisions.”

Facebook Twitter Pinterest Pius XII in 1951.

Pius – whose birth name was Eugenio Pacelli – was Vatican secretary of state under his predecessor, Pope Pius XI, and a former papal nuncio, or envoy, to Germany. In 1933, he negotiated a concordat between the Catholic church and Germany. After he was elected pope, six months before the outbreak of war, the Vatican maintained diplomatic relations with the Third Reich, and the new pontiff declined to condemn the Nazi invasion of Poland on 1 September 1939.

In December 1942, Pius XII spoke out in general terms about the suffering of the Jews, although he had known for several months about the Nazi extermination plans. In 1943, he wrote to the bishop of Berlin, arguing that the church could not publicly condemn the Holocaust for fear of causing “greater evils”.

Hitler’s Pope, a controversial biography of Pius XII by British author John Cornwell, published in 1999, claimed the pope was an antisemite who “failed to be gripped with moral outrage by the plight of the Jews”. He was also narcissistic and determined to protect and advance the power of the papacy, the book argued. Pius XII was “the ideal pope for Hitler’s unspeakable plan. He was Hitler’s pawn. He was Hitler’s Pope.”

Cornwell’s claims were challenged by some scholars and authors. He later conceded that Pius XII had “so little scope of action that it is impossible to judge the motives for his silence during the war”, although the pontiff had never explained his stance.

In 2012, Yad Vashem changed the wording on an exhibit on Pius XII’s papacy, from he “did not intervene” to he “did not publicly protest”. The new text acknowledged different assessments of the pope’s position and Yad Vashem said it “look[ed] forward to the day when the Vatican archives will be open to researchers so that a clearer understanding of the events can be arrived at”.

Pope Benedict, Francis’s predecessor, declared in 2009 that Pius XII had lived a life of “heroic” Christian virtue, a step towards possible sainthood. But in 2014, Francis said no miracle – a prerequisite for beatification, the final step to canonisation – had been identified. “If there are no miracles, it can’t go forward. It’s blocked there,” Francis said after visiting Yad Vashem. Last year, Francis said Pius XII had led the church during one of the “saddest and darkest periods of the 20th century”. He added that he was confident that “serious and objective historical research will allow the evaluation [of Pius] in the correct light,” including “appropriate criticism”.