Italian Maria Crocifissa Della Concezione wrote the message back in 1676 – claiming the devil had controlled her.
Scientists from Ludum Science Centre in Catania, Sicily, have finally decoded the message.
And its contents are spooky, describing God, Jesus and the Holy Spirit as “dead weights”.
Maria entered the Benedictine convent in Palma di Montechiaro at the tender age of 15.
One morning she woke up to discover the mysterious letter.
She claimed she had been possessed by the devil and was forced to write it.
The nuns believed her and displayed the letter at the convent, hoping someone will be able to make sense of its code.
But it lay without explanation – until now.
Using code-breaking software found on the dark web, the team confirmed its devilish message.
One sentence reads: “God thinks he can free mortals.”
Another says God was invented by man and that “this system works for no one”.
It goes on to say: “Perhaps now, Styx is certain.”
This is believed to reference the River Styx that separates the Earth and the Underworld in Greek and Roman mythology.
Daniele Abata, director of the center told the International Business Times: “We heard about the software, which we believe is used by intelligence services for code-breaking.
“We primed the software with ancient Greek, Arabic, the Runic alphabet and Latin to unscramble some of the letters and show that it really is devilish.”
The scientists say the letter is not entirely consistent or understandable.
This supports the theory that Maria suffered from schizophrenia or bipolar disorder.
Yesterday it was revealed Jack the Ripper sent postcards to cops.