Now - 361 years later - researchers at the Ludum Science Centre in Sicily claim to have decoded 15 lines of the only surviving letter using a piece of decryption code they found on the Dark Web.
"Everything's on there: drugs, prostitution, paedophilia, and also programs used by intelligence services to decipher secret messages, like the one we used," Ludum director Daniele Abate told Italian radio 105 Network.
"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."
The scientists concluded that the letters were a jumble of languages and were composed and written by Sister Maria herself; she had become a skilled linguist during her time at the convent.
The letter describes the relationship between humans, God and Satan in a rambling and inconsistent manner. In it, Sister Maria - or whoever had possessed her - encouraged God to abandon man and leave him in the clutches of the devil.
"God thinks he can free mortals, this system works for no one," one of the translated lines reads. The text also describes God, Jesus and the Holy Spirit as "dead weights".
Ms Abate said the contents of the letters made her suspect Sister Maria may have been suffering from schizophrenia.
"I personally believe that the nun had a good command of languages, which allowed her to invent the code, and may have suffered from a condition like schizophrenia, which made her imagine dialogues with the Devil," she told The Times of Israel.
"That has not stopped numerous interested Satanic sects contacting me since I published our findings."
