It was also mentioned in the letter that Tilly was trying to translate the characters and intended to see if the Witch Cooperation Association had any idea.
Roland decided to reply the letter with the updates about the demon and Holy City of Taquila along with the picture of restored scene Soraya drew, which might help her on her translation. The information like this was not supposed to be hidden. If they could figure out the misfortune 400 years ago sooner, they would cope preparation sooner and might discover the weakness of the church. If the church had nothing to hide, they would not have tried so hard to cover the history.
There was apparently something they were trying very hard to hide.