Maybe a version of scripture that had been "rejected" by some King, and was illegal to reproduce? Take the best radiocarbon dating, figure out who was King back then, and if they 'sanctioned' any biblical translations, and then go to the version of the bible before that translation, and this will be what was perhaps illegal and needed to be encrypted. That's just one plausible story. Who knows, we might find out the phrase "young girl" was simplified to "virgin", and that would potentially be a big secret.