Most of the sources I have found address reading books or listening to music by non-Jews. Films are a different issue, because they can often be more immodest than the books they were based on. I don't know whether films could necessarily be compared to books, even if they are modest (one concern that comes to mind are the more powerful impressions they leave, as opposed to books, because they play with more of your senses). So I'll be addressing the books:
Should these books/movies be avoided on the basis that the authors were Christians? If so, what about everything else written by authors who are part of other religions?
I have not seen anyone suggest that books should be avoided simply on the basis that their authors were Christian, or any other sort of non-Jew.
Rabbi Shmuel Ariel wrote regarding reading books written by non-Jews (my translation):
"...[we have not found in halacha any prohibition of reading] all books written by non-Jews, from a fear that ideas related to idolatry and heresy may be mixed in. It is prohibited to read books that contradict emunah, but it is not prohibited to read a book written by a someone who is not a believing Jew..."
I recommend reading his full teshuva if you are able (it's in Hebrew).
Rabbi Ilay Ofran also wrote similarly that as long as the material was proper (modest), then it would be allowed. Not reading something written by a non-Jew simply because the author is not Jewish may be regarded as potentially racist.
Is it a content issue? At face value the Narnia books and movies don't seem to directly say anything about Christianity, even if the author did intend it. And Tolkien apparently did not intend Lord of the Rings to be religious at all, although some say it contains Christian themes (which seems like a stretch to me). Where should we draw the line?
Yes-- to a certain extent:
Rabbi Gil Student made a post on the subject of reading the Chronicles of Narnia. He argued that though Narnia is heavily-laden with Christian themes, it is not obvious unless you are very familiar with these themes and/or have the issue pointed out to you (I humbly agree, from my own experience, which is similar to that of Rabbi Student). Therefore, reading these books does not qualify as studying Christian theology.
"Regardless of the halakhic details — meaning, even if you can find ways around the possible issues discussed above — my rabbi and I agree that reading these books and seeing these movies are certainly not recommended. Stay away from other religions in your entertainment needs. There’s plenty of other ways to relax and enjoy yourself without having to partake in subtle Christian allegories."
Rabbi Shai Piron was asked about Sci-Fi/fantasy novels and films both in general, and as an example Lord of the Rings was mentioned. He answered in general terms and I think his point is important. Here's a translation of the Q&A:
"Q. For a while now I've been hearing various opinions on the subject of reading Sci-Fi novels and watching their films (if there are such)
The main issue is regarding books that feature magic and sorcery such the "Lord of the Rings" trilogy and the Harry Potter book series.
My question, is whether those that say it is not permissible to read these books have a halachic basis?
And as one who really enjoys reading these sorts of books-
Is it allowed? What's the problem with that? And why exactly does this problem exist?
If you regard these books as reflecting reality, it is certainly not allowed. There is a great danger of idolatry and I do not see any way to permit such a thing. But if you simply enjoy stretching your imagination via these books, from the powerful descriptions and the ability to create films with so many special effects - there is no prohibition."
Rabbi Ariel further noted in his answer some general points regarding judging what material could be read and what couldn't:
- Novels or history books where idolatrous practices are mentioned in passing, as things that simply occurred, and not in a manner intended to glorify these books are not prohibited.
- The same is true regarding books that borrow quotes from idolatrous works. If it is presented as something someone said and not intended to glorify it, then it is not prohibited.
- This is even more true when the author himself does not believe in these things.
An example that comes to mind, particularly of the third point, are the Cosmere novels by Brandon Sanderson. Sanderson himself is a Mormon, but his books often present various fictional idolatrous religions and cults. Setting aside whether his novels have Mormon themes, it is clear that he himself does not believe in any of the religions he made up.
To sum up their points, in general - these books are not prohibited. However, one should take care to see how the material is presented and what you, the reader, are taking away from the book.