I've always thought this Lewis quote was particularly apt:
"But in general, take my advice, when you meet anything that’s going to be human and isn’t yet, or used to be human once and isn’t now, or ought to be human and isn’t, you keep your eyes on it and feel for your hatchet."
(Mr. Beaver, on the White Witch, from "The Lion, The Witch, and the Wardrobe")
I don't especially love Lewis. His fiction is fine, but I find his theology badly overrated. At best it's pop theology, not scholarly, and even even as pop theology I find it shallow.
Ironically, I find his friend Tolkien's Catholic theology more thoughtfully expressed in fiction that Lewis' is in nonfiction.
Generally we restrict the notion of lying to mean willful assertions that are known to be false. But there are other problems here. We tend to divide knowledge into the categories of a posteriori (that is which justified belief through observation and experience) and a priori (that which is justified belief through structural consequence, or without experience). C.S. Lewis being a Catholic is making an assertion on an a priori facet of knowledge.
Anything is true provided you take the right postulates. You should always keep that in mind, as well as the fact that presuppositional critique on good faith belief is an uninteresting game of semantic bickering, and as a baseline itself requires epistemic certainty. Or rather, "you can't assert that because we don't know" isn't really a valid attack here. You might not know, because that lack of knowledge is entailed by your world model. C.S. Lewis knows because it is entailed by his world model.
"But in general, take my advice, when you meet anything that’s going to be human and isn’t yet, or used to be human once and isn’t now, or ought to be human and isn’t, you keep your eyes on it and feel for your hatchet."
(Mr. Beaver, on the White Witch, from "The Lion, The Witch, and the Wardrobe")