As several folks here have observed a few times, sermons thinly disguised as fiction suck. And not in the fun way, either.
Personally, I’ve never enjoyed a message tract with a thin veneer of fiction, regardless of whether or not I agreed with the message. These can work reasonably well in short form, if written well enough (very few authors meet that bar), but in novel-length works, not so much.
The end result emerges rather like the pizza the Husband and I once ordered from the local pizza joint, where we made the mistake of requesting extra garlic. What we got would have given any self-respecting vampire fits: we couldn’t taste anything except garlic. We took a bite, looked at each other. The rest of the pizza went in the trash. The words “I wanted a pizza with extra garlic, not garlic garnished with pizza” were used. Then we found something else to eat, and we never went near that place again. Even though at the time it was quite literally half a block away.
This is what message fiction usually does to the reader who isn’t reading it for the message (someone who’s reading for the message is likely to get irritated by the unnecessary plot and characterization the author has added, so it’s a no-win either way). It turns them off. Sometimes it turns them off reading altogether, especially when they’re force-fed a diet of the most dreary, dismal, and shitty message fiction imaginable (hello, school reading lists).
I’m sure the proponents of having fiction with the message would think this is not a bad thing, but I beg to differ. You see, readers of fiction tend to draw their own messages from that fiction. It was fiction that taught me it was possible to endure and emerge more or less intact despite years of vicious bullying. Fiction gave me hope, and it showed me there were ways to be who I was even if things were shitty at the time.
It wasn’t message fiction. It was a mix of things: any historical fiction I could get hold of in the school and town library (and that they’d let me borrow, since librarians tend to be kind of reluctant to let the 8 year old kid borrow from the grown-ups section), Doctor Who novelizations (which lead into my love of science fiction and fantasy), and pretty much anything else that took my fancy. I read so much that I didn’t have a library card, I had a set of them stapled together and I used the initials of the book title to write in what I was borrowing because I was getting them in job lots.
Along the way I picked up an extra serve of Heinleinian cussed independence, some of Laura Ingalls Wilder’s self-sufficiency, an abiding loathing of bullying in any form, and many, many reminders that I could do good and sometimes heroic stuff too, even if I was scared.
If I’d been stuck with message fiction, I wouldn’t have gained any of that, not least because the characters in every message piece I’ve read are functionally ciphers standing in or sometimes embodying the virtues the author wants to showcase or the evils the author wants to decry. I preferred the heroes who were scared and did it anyway because they had to. Edmund Pevensie, confused and frightened, and realizing what he’d betrayed but still going after the White Witch and nearly dying in the process. Jill in a later Narnia book crying and trying to keep her bowstring from getting wet because she couldn’t afford to do that. Laura Ingalls and her battles with jealousy of her too-perfect older sister who she also loved dearly (Yes, I’m aware the Little House books are fictionalized autobiography. I didn’t know that when I read them as a child). Anne Shirley and her often disastrous romantic fantasies.
The messages came through without the sermon. Simple messages: it’s better to be honest than not. It’s better to be kind than cruel, but sometimes you have to be harsh and sometimes the wrong group wins. Life is harsh and life doesn’t care. Be true to yourself. Those messages.
Perhaps more to the point, I learned them for myself, without some Great Expert telling me how it was supposed to be. It’s because I learned them for myself that they stuck and they meant something to me. Being lectured is just like being stuck in a classroom waiting for the bell to go so you can be free again.
Since most people want to do things their way at some level, letting message emerge seamlessly from the interaction of your characters and plot for your readers to discover has much more impact. It also doesn’t make reader-puppies sad and desperate to escape you.
I know which way I choose.