This requirement is not about having Christian themes or general concepts. There are many requirements and features which fall under those categories but should not be categorized under this requirement.

This requirement is about having direct Biblical parallels to particular Biblical passages.

Part of the motivation for this requirement is to not simply have a book which sounds like it was written by someone who heard about the Bible secondhand, but a book which is directly inspired by the Bible.

Prick him anywhere; and you will find that his blood is Bibline, the very essence of the Bible flows from him. He cannot speak without quoting a text, for his soul is full of the Word of God. - C.H. Spurgeon, speaking of John Bunyan