Back in the 17th century, a Sicilian nun wrote a letter claiming she had been possessed by the devil. Over 340 years later, scientists have finally deciphered this rambling message using a decryption program they came across on the deep web.

The letter was supposedly written by Sister Maria Crocifissa della Concezione at the Monastery of Palma di Montechiaro in the early hours of August 11, 1676. The following morning, she awoke covered in ink and claimed she had been possessed by Satan, who forced her to write the message. At the time, claims like these were taken very seriously. However, no one could work out what the letter said because it was written in a rumbled archaic alphabet, Italian news Radio 105 reports.

There have been numerous attempts over the centuries to decode the letter. In the 1960s, the monastery even offered a month-long vacation to anyone who could successfully translate the message.

Now, the letter has made its way into the hands of Ludum Science Center in Catania, who have been deciphering the symbols using a code-breaking software found on the deep web.

“We heard about the software, which we believe is used by intelligence services for codebreaking,” Daniele Abate, director of the Ludum Science Center, told The Times. “We primed the software with ancient Greek, Arabic, the Runic alphabet, and Latin to de-scramble some of the letter and show that it really is devilish.”

They have already translated 15 lines of the letter. So far, their work has revealed that the letter speaks of the relationship between God, Satan, and humans. It reads: "God thinks he can free mortals. This system works for no one... Perhaps now, Styx is certain." It goes on to try and convince the nun to abandon her faith, arguing that God is merely the invention of man and that Jesus and the Holy Ghost are “dead weights”.

Still, the question remains, why or how did the nun manage to write this? Well, first of all, the researchers have reason to believe the woman was suffering from bipolar disorder, so she could be prone to experiencing delusions or hallucinations. She also was familiar with several languages and spent a lot of her time studying linguistics.

"My theory is that this is a precise alphabet, invented by the nun with great care by mixing symbols she knew," Abate told Radio 105. "Each symbol is well thought out and structured, there are signs that are repeated, perhaps an intentional and perhaps unconscious initiative. The stress of life in the monastery was very strong."