I don't quite understand the meaning of the phrase "there is to know."
At first glance, I failed to figure out why the phrase should be here, because "everything about God" appears to be sufficient to convey what the author wanted to tell the reader.
On second thought, the present tense of the phrase appears to be necessary because it compares the present situation to the 14th century. The implication is that for this revived man, while his knowledge about science is a disaster, his knowledge about God is as perfect as that of best Christians today. Am I on the right track?
If so, I am still not very clear why "to know" should be used. "Everything there is (presently) about God" seems okay to me.
The best guess I've now got is that the structure here is "know...everything there is to know..." (I understand it as "know...everything one should know that exists presently...", so "to know" means "one should know"). I am not very sure.
Imagine that we could revive a well-educated Christian of the 14th century. The man would prove to be a total ignoramus, except on matters of faith. His beliefs about geography, astronomy, and medicine would embarrass even a child, but he would know more or less everything there is to know about God.
Source: The End of Faith by Sam Harris p.22