I haven’t read everything he ever wrote, but I think that most of his books are just fantasies, or perhaps early science fiction (Out of the Dark Planet). Many of them are allegorical; you might see parallels between them and concepts or elements from the Bible, or you might ignore that and just enjoy the story. I think you could enjoy The Screwtape Letters in the same way you would enjoy Good Omens, as a fantasy that has demons in it.
the whole point of the screwtape letters is defending christianity. I know it's wikipedia, but wikipedia says it's christian apologetics in the first sentence, and goodreads says it's religious satire in the first sentence.
