Vatican archives documenting centuries of European history including Galileo Galilei's trial documents and Martin Luther's excommunication went on public display for the first time Wednesday.
The exhibit also includes a request to annul Henry VIII's marriage to Catherine of Aragon and the 'Dictatus Papae' of Pope Gregory VII, an 11th-century script asserting the spiritual and terrestrial powers of the head of the Roman Catholic Church.
The exhibit entitled "Lux in Arcana" in Rome's Capitoline Museums will run until September 9 and organisers said it was a unique chance to see a priceless collection of documents from the Vatican's closely-guarded vaults.
"It will be the first and possibly the only time in history that they leave the confines of the Vatican City walls," organisers said in a statement.
They said the show has "100 original and priceless documents selected among the treasures preserved and cherished by the Vatican Secret Archives for centuries" and includes multimedia installations about the documents.
The exhibition marks the 400th anniversary of the creation of the Vatican Secret Archives -- a term used to mean personal archives -- by Pope Paul V.
The Holy See's second in command, Secretary of State Cardinal Tarcisio Bertone, opened the show with Vatican culture "minister" Gianfranco Ravasi, Rome mayor Gianni Alemanno and Italian Culture Minister Lorenzo Ornaghi.
Asked what he had been most struck by, Bertone said it was the "historical truth" in documents on Pope Pius XII's papacy during World War II, who has been criticised by Jewish groups for not doing enough to ease their persecution.
"The research on the period of Pius XII has so far generated more than two million files and information about prisoners of war," he said.
One of the documents in the exhibition is a report from papal envoy Francesco Borgongini-Duca on the conditions in seven internment camps in Italy in 1941, asking for aid to be sent to the prisoners.
At a press conference on Wednesday, the head of the Vatican archives Sergio Pagano said all the documents in the vaults from Pius XII's papacy would be made available to researchers "within one or two years".
"The final decision however depends on the pope," he told reporters.
"Benedict XVI's willingness to accelerate the opening, also as a way of silencing dissonant voices on the pontificate of pope Pacelli (Pius XII), can only benefit the Church," he said.
Bertone said earlier the exhibition would help "the search for truth and the common good" -- and would dispel "a pseudo-historical novelistic ambiance", an apparent reference to Dan Brown's bestselling "Da Vinci Code".
Among other treasures are a 10th-century parchment on the division of powers between pope and emperor and a document on the nomination of 13th-century hermit Pietro Morrone as Celestine V -- the only pope ever to resign.
It also has minutes from the 14th-century trials of the Knights Templar.
There is also a 15th-century edict from pope Alexander VI on carving up the New World between Spain and Portugal after Columbus's discovery of America, as well as a secret code he used when he was besieged by French troops.
There are letters from Michelangelo about building St. Peter's basilica in the 16th century, the deed of abdication by queen Christina of Sweden from 1654 and a letter on silk from the 17th-century Chinese empress Helena Wang.
Among the most unusual documents is a letter written on birch bark from the chief of the Ojibwa Native American tribe to pope Leo XIII in the 19th century, calling him: "Grand Master of Prayers, who makes functions of Jesus."
Another rarity is a letter from imprisoned French queen Marie Antoinette after the revolution in 1789, which reads: "The feelings of those who share my sorrow... are the only consolation I can receive in this sad circumstance".
Rome's mayor Alemanno said: "This exhibition is really unique and exceptional. This is the first time that the Vatican secret archives open their doors for an incredible exhibition that spans all historical eras."