The Eiður is the holy book of The Heirloom, the god of all things that are good in the world. It is renowned as one of the best written religious texts in the world, mostly due to the fact that no other holy text has been found such as this to be both so widely distributed but also fully legible.
It is said that The Heirloom of Eiður magically created this text to push upon his word to the people of Rakuen. It is likely the purpose of this text to use those who are devout and literate to spread his word upon the simplest vector, word of mouth, so that more people will hear his name and take on his mantle of bringing goodness and kindness to the world.
The original discussion is spoken word, and thus in a straightforward form. The written form, as transcribed by clergy, tends to separate it logically into parts of the document to make sure that it is appropriately spaced out and easier to read.
Because this is seen as a pure religious teaching directly from The Heirloom to some of his most ardent followers, this text is seen as a pure holy document. Although it carries no historical clout beyond the fact that it exists, many nations will use chapters or phrases to justify good deeds that they have done at given times to insist the things that they are doing are in the best interest of their own nations, whether they actually are or not.
The public tends to view this document favourable, proving the reality of the Eidolons without question. Many trust its preaching and abide by them as best they can. Others claim that this is a form of mind control for the masses, and that various government co-opt mistruths about religion to try to sway public opinion.