As the confidential correspondence of popes, princes and potentates, they have been jealously guarded for centuries.
Now 100 of the most historically significant documents held by the Vatican's Secret Archives are to go on public display in Rome -- the first, and probably last, time that they will leave the buttressed stone walls of the tiny city state.
The priceless documents span more than a millennium, from the eighth century to modern times, and feature a cast of historical characters ranging from the Knights Templar to Galileo, Martin Luther and Henry VIII.
They are normally kept on 50 miles of shelves in climate-controlled rooms in the Vatican's Apostolic Palace as well as in a high-security underground bunker.
Archivists have gathered them together for an exhibition, to be held in Rome's Capitoline Museums to celebrate the 400th anniversary of the founding of the Secret Archives in their present form.
"It's an exceptional event," said the Vatican's spokesman Father Federico Lombardi.
"Never have so many documents from the Secret Archive been allowed to leave the Vatican."
Organisers of the exhibition said it will "recount history through its sources".
One of the most interesting documents for British visitors will be a letter written by Mary Queen of Scots a few weeks before her execution for alleged involvement in plots against Elizabeth I.
Mary wrote the letter in French to Pope Sixtus V from Fotheringay Castle in Northants, in November 1586.
It rails against the falsehoods perpetrated by her enemies in England and claims that the tribunal that condemned her to death was illegitimate. It also recounts the suffering she had undergone during nearly 20 years of imprisonment.
An appeal by Parliament asking the Pope to annul Henry VIII's marriage to Catherine of Aragon will also be in the collection.
The parchment document was sent to Pope Clement VII in 1530 but failed to resolve the dispute, which eventually led to a schism and the founding of the Church of England.
It will be displayed alongside documents from the trial of Galileo Galilei, whose theory that the Earth revolved around the Sun was regarded as heresy by the Roman Catholic Church in the 17th century.
The exhibition, 'Lux in Arcana: The Vatican Secret Archives Revealed', will run from February to September next year. (© The Daily Telegraph, London)