In pictures: secrets of the Vatican archives

The term 'secretum' has been used since the 15th century for people or institutions close to the authorities who could be trusted. The Vatican's secret archives, the 'bibiloteca secreta', are the personal, or private archives of the pope Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The Pax Veneta was signed in 1177, and declared a six-year truce between the Lombard League and Frederick Barbarossa, the Holy Roman Emperor. This document is a copy, bearing the signatures of the emperor's representatives on the left-hand side Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The Caliph of Morocco, Abu Hafs Umar Al-Murtada, wrote to Gregory VII in 1250, requesting the appointment of a new bishop in Fez of the 'same clear spirit, and good conduct' as bishop Lupo, who delivered this letter to the pope on his return. As was customary, the letter begins with the invocation 'In the name of God, the clement and merciful', which is followed by a blessing on Muhammad Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The French Inquisition arrested all Templars resident in France in 1307, tortured them and extracted confessions of heresy. Clement V launched his own investigation in 1308, which culminated in an interrogation of the Grand Master of the Templars at Chinon. This document, of which only a detail is shown, pronounces the canonical absolution of the order. Philip the Fair of France reacted by increasing his attacks on the Templars, and four years later the order was dissolved Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

After the plague of 1348 which inspired the Decameron, Giovanni Boccaccio became more reflective, and underwent a moral conversion, possibly under the influence of his friend Francesco Petrarca. A decision to take minor orders required this papal dispensation, issued in 1360, because of his illegitimate birth Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

For 30 years at the end of the 14th century the western church was divided, with first two popes and then three elected by conflicting sections of the Sacred College. This solemn letter to the community of Corneto reports the resolution of this struggle, with the abdication of Gregory XII, the deposition of John XXIII and Benedict XIII and the election of Martin V Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

This door is on the second floor, which holds the Holy See's diplomatic correspondence. Missives from legates, nuncios and other agents are kept in the special wooden cabinets ordered by Pope Alexander VII in 1660, which are still in use today Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

This codex is a compilation of material drawn from a collection of documents put together by cardinal Nicolas Rossell. The manuscript probably dates from the second half of the 15th century, and may have been commissioned by Cardinal Rodrigo Borgia. It is the work of two copyists and a miniaturist, who decorated almost every page Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The discrepancy between the Julian calendar and the observations of astronomy had become unbearable by 1576, when Pope Gregory XIII launched a special commission to investigate how it might be reformed. The commission adopted the proposals of Luigi Giglio, who suggested suppressing 10 days, so as to realign the dates of the astronomical and canonical equinoxes. This new calendar was adopted in 1582, with the loss of 10 days in October. The Gregorian calendar was adopted the following year in Italy, Spain and Portugal. Great Britain adopted it in 1752, Russia in 1918 The Gregorian Calendar and the Lunario Novo of 1582 Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The Papal Bull which condemned Martin Luther was published on 1520, and met with stiff resistance in Germany. In Leipzig the Bull was covered in mud and lacerated. At Erfurt, theology students tore it up and threw it into the river. Luther himself burned it along with the Canon Law Books at Wittenberg. Given the near-total absence of originals, the transcribed text of the Bull in the registers of Leo X shown here becomes all the more important Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

A view of the Basilica of St Peter in Rome, seen through the columns of St Peter's Square Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

This letter, by the 14-year-old Lucrezia Borgia, was written to her father, Pope Alexander VI, in 1494. In it she tells of the reception she and her new husband, Giovanni Sforza, received on their arrival in the city of Pesaro, and begs her father to leave Rome Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

This letter from Ascanio Colonna, written in 1526 to one of his lieutenants, evokes the battles between the forces of Clement VII and the Colonna family after the sack of the Vatican in September Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

Clement VII received this letter from the peers of England concerning the marriage of Henry VIII and Catherine of Aragon in September 1530. The pope was struck by its peremptory tone, the nobles complaining of the excessive delay in issuing an annulment and suggesting that a refusal would result in extreme measures being taken for the good of the kingdom Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

This sumptuous document is a letter from Clement VII on the occasion of the coronation of the Emperor Charles V in 1530. Lavish decorations are paired with a seal made of gold, rather than the usual lead Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

Banned by the French authorities after just three performances, Voltaire attempted to relaunch his play Mahomet ou le fanaticisme with a dedication to noble personage: Pope Benedict XIV. He declared himself an ardent admirer and requested a portrait, which he hung in his study. Writing in August 1745, he expressed his thanks, and joked that he 'implore[d] Heaven that Your Holiness might be most tardily received among those saints whose canonisations you have so laboriously and successfully investigated'. Voltaire published the exchange of letters soon after, unleashing a storm of outrage Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

Pius IX launched a process to define the dogma of the immaculate conception with a letter to his bishops in 1849. Five years later, the Bull Ineffabilis Deus was published, declaring that 'the most Blessed Virgin Mary, in the first instance of her conception ... was preserved free from all stain ... a doctrine revealed by God and therefore to be believed firmly and constantly by all the faithful' Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The Tower of the Winds was richly frescoed by Nicolò Circignani along with the brothers Matthijs and Paul Bril between 1580 and 1582 Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The Grand Vizir of the Ottoman Empire, Karà Mustafà, attacked Vienna with an army of 160,000 men in 1683. Appointed as head of the Christian army by the intervention of Pope Innocent XI, the King of Poland, John Sobieski, led an army of 70,000 men to the walls, and broke the siege on 11 September after eight hours of combat Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

After receiving a letter from Adolf Hitler, seeking an improvement of relations between Germany and the Vatican, Pope Pius XI replied in 1934, assuring him that the Holy See would continue to promote 'true peace and concord between civil and religious authorities' Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books

The long gallery, west of the Cortile del Belvedere, holds more than 13,000m of metal shelving. The archives of the Apostolic Datary, the Vatican Information Office, the Central Commission for Sacred Arts in Italy and the Audience Sheets of the Prefecture of the Papal Household are among the documents kept here Photograph: The Vatican Secret Archives, published by Paul Van den Heuvel - VdH Books