Sister Maria Crocifissa della Concezione wrote 'this system works for no one' and that humans invented God in the rambling letter.
She believed she had been taken over by the devil who was using her hand to write with in 1676.
'God thinks he can free mortals,' it reads, then refers to the river of the underworld saying 'Perhaps now, Styx is certain'.
The letter was found at a convent in Sicily, where the nun had lived since she was 15.
She claimed she woke up one day and found the letter, with no recollection of who wrote it.
Scientists from the Ludum Science Centre in Catania finally managed to translate the letter, after running it through software on the dark web which is used by intelligence services to break codes.
It uses Latin, Ancient Greek, the runic alphabet and Arabic to try and make sense of the characters – and came up with some decipherable phrases.
Researchers said the nun was known to scream and faint at the altar of Palma di Montechiaro convent, saying the devil was trying to make her serve evil rather than good.