You correctly describe what the quote literally says, but there's a fine line between "I'm just writing a story which requires that these particular materialists be biased" and "I'm accusing materialists in general of being biased like this".
Lewis is not accusing materialists in general of having that bias - Lewis is accusing humans in general of having that bias. The idea that humans have that bias, and that that bias can be exploited to convince a human to subscribe to a given philosophy independantly of whether that philosophy is true is rather the point of the quote.
Well, I think it is both.
Lewis described how humans can subscribe to a given philosophy independently of its correctness. That part is completely rational.
But he didn't use a random example to describe this bias. -- Just look at all other books he wrote, they all deal with the same topic. His choice of Devil is not the same as e.g. Tolkien's choice of elves. Tolkien wrote fiction, but Lewis wrote fiction as a propaganda tool. Tolkien didn't believe in elves, but Lewis did believe in Devil. -- Therefore it seems to me very likely that he wanted his readers ...
Another month has passed and here is a new rationality quotes thread. The usual rules are: