I'm not trying to guess why you would say that (and by that, I mean the fact that you would avoid writing because it's trying to preach to you), and I do want to say that Lewis is, overall, very close to proselytizing in his writings (generally it's pretty explicitly stated that's what he wants to do). But.

Avoiding something because it disagrees with your worldview, or because it is 'too close to [insert literally anything here]' is not a great way to expand your understanding of the world. Being exposed to things you disagree with is step 1 in understanding how or why they exist, with few exceptions (CP, explicit hate speech, most calls to violence, those sorts of things).

Read the screwtape letters. Yes, they're intended to make you consider your immortal soul and embrace Christianity. But also yes, it's an amazing piece of literature.

Not sarcastic. The Screwtape Letters is important literature because it was praised at publication, became lauded, and is an complete intellectual failure. The Screwtape Letters stands as a milestone of logrolling: the practice of approval due to who authored something rather than what the writing actually contains. The Screwtape Letters is a debate exposing one of the several weak intellectual foundations of Christian philosophy - but none of that critism was socially allowed for many decades. Such critism still causes many a panty to bunch. The Screwtape Letters is important because it is weak intellectualism, wrapped up in pretty good writing, but not good enough to disguise its intellectual bankruptcy, yet it still lauded.

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.

I mean he was a Christian and he did believe strongly in it and tried to share his beliefs with others. I'm not sure if I would call that zealotry though.

Not a joke; the Screwtape Letters is a seminal piece of literature, forget the written story, the story is the context of the novel, it's intention, it's composition, and it's ultimate weakness as a intellectual milepost. It practically destroys itself. It's a masterpiece of self deception.