Over the years, I’ve encountered folks who were uncomfortable with some of Lewis’s Narnian mythology because of it’s inclusion of overtly pagan mythological characters and deities as being on the GOOD guy’s side.
A grumpy faun doesn’t look that much different from some representations of the devil. Dionysus (Bacchus), the god of drunken orgies (among other things) is in there. So is Santa Clause.
The same could be said of the druid Merlin in That Hideous Strength, thought he does go out of his way to explain that a bit more.
Along those lines, is this interesting note about some of Tolkien’s original criticisms of Lewis’s fantasy:
In his article “J.R.R. Tolkien: Narnian Exile,” Joe R. Christopher suggests that Lewis responded to Tolkien’s criticism of the first two chapters of The Lion, the Witch and the Wardrobe by adding a section to the end of Prince Caspian. He argues that one of Tolkien’s major objections is that Lewis sanitizes or sentimentalizes mythical creatures, taming (and thereby misrepresenting) the nature of characters like the faun or satyr. Late in Prince Caspian, there is a wild romp of mythological characters, including Bacchus and Silenus. In the story, Susan observes, “I wouldn’t have felt very safe with Bacchus and all his wild girls if we’d met them without Aslan”. As these mythological creatures become part of this story, Lewis argues, their behavior is redeemed. Christopher explains, “That is, Lewis seems to reply to Tolkien, under Christ certain basic impulses can be controlled…under Christ, such things can be kept in bounds”. Christopher says of this passage, “It is difficult not to believe that this is a deliberate answer by Lewis to Tolkien”.
-Diana Gyler, The Company They Keep, p.113