A little background here...
George MacDonald was an 18th-century Scottish pastor. CS Lewis has said that in his book, The Great Divorce, and many other works, George is credited as his biggest mentor for Christianity. Lewis, as an atheist, was harsh against Christians because he saw Jesus in the fairy tales and the myths of old and concluded he was made up. George's fantasy works and Tolkien's use of myth as a guide ultimately turned a hardcore atheist towards Jesus.
To paraphrase Michael Heiser, a biblical scholar, on the UFO people, they are predisposed to a good theological discussion because they are already asking these questions. Those in the fantasy world have a similar take, as the elements within fantasy are usually half real and half spiritual, asking the same questions.
George MacDonald, in an essay on why, as a pastor, he wrote fantasy, said it's the one vehicle you can use to explore the biggest ideas and questions because it doesn't limit you to a box. This is me paraphrasing his essay in a few words. Lewis took what George did and ran with it to showcase Christ's attributes and wonder, which are at the heart of the Bible, and draw us towards Jesus.
Tolkien used his background to create a world to showcase the morals we face in good vs evil, using fantasy tropes while staying true to his faith. Gandalf was never God or Jesus as some will try to argue angrily against Tolkien. He was an angel sent to help men. This is why he goes by different names, as he is described in The Silmarillion.
Putting all that aside, the church has done a huge disservice to condemn fantasy in general since it fails to understand what magic is, and because of this, it has run witch hunts on modern authors, claiming they follow Satan. You might as well claim God is evil for telling Israel to USE pagan magical practices! Oh the shock! LOOK at some of the Levitical laws or how Israel was told to use divination. It's the same practice in pagan circles. The issue here was loyalty. Who's God were you loyal to and who were you asking questions of? The ephod on an Israelite priest’s robe? That's also something done in the pagan world for the exact same reason. Technically, this would be called magic, as it involves the spirit world and God.
Personally, I stay clear of certain authors because I know they use real magic in their writing. The issue is loyalty. What is the real message of the story and how is this magic used and portrayed? I know exactly how Tolkien and Lewis wrote their magic system and it does not break any laws of the Bible. If magic is the mystery of the unknown, today's technology would be magic to the writers of the Bible in a sense. But if you practice rituals and incantations, calling on the spirits to do your bidding, the biblical authors would know instantly what this is and condemn it immediately.
NONE of that is in Tolkien's work or CS Lewis's books. If you look at the backstory of Lord of the Rings, you'll get a clear indication of what is going on. Louis Markos, a Christian literary professor, wrote in an article for The Gospel Coalition, a powerful message showing how Tolkien's work so easily mapped the Bible.
To claim we shouldn't be reading anything but what affirms biblical principles really has a glaring error to it. Namely, we KNOW the authors of the Bible read pagan books! Otherwise they wouldn't be writing the stuff they did that clearly addresses these very things. I mean, even David QUOTES part of the Baal Cycle in Psalms to make a point! If you're going to argue on this, you're going to find yourself against a wall with the Bible.