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To convey an underlying message, "X is better than Y", you need to present your "argument" in terms of scenes and characters and have the outcomes for these characters prove the point. For example...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/33195 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/33195 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
To convey an underlying message, "X is better than Y", you need to present your "argument" in terms of scenes and characters and have the outcomes for these characters prove the point. For example, IRL I believe Nordic-style socialism is superior to Western-style free-market capitalism. Now I am well-studied extensively on both, so I can list twenty failures of free market capitalism and twenty huge advantages of Nordic-style socialism. If I wanted to put that argument into a fantasy novel, I would **illustrate** the differences with characters trying to pursue their dreams, and failing under free-market capitalism, or doing active harm to succeed. While other characters, under Nordic socialism, do not fail, and succeed without harming anybody, and in truth actually help others to succeed. I don't have to tell you "X is better than Y", because in the story, Y leads to failure and X leads to success. You don't club people over the head: They may not even figure out this is what the author wanted to say! Because when immersed in a story readers _forget_ there is an author and read the events and outcomes as "what actually happened" (in this story universe) and, unless you stray into reverie-breaking implausibility, it doesn't occur to them this outcome is **chosen** by you and **engineered** by you for 250 pages to be how the story turns out: X prevails and kicks Y in the face. And that can happen multiple times in the story, in one form or another, as part of character histories and stories they tell, in the news and politics they hear, in your exposition. You will have to choose an appropriate story, of course. Detailing the difference in governance between the USA and Norway may not be a discussion to achieve in a story of rival gangs in Chicago. Competing cultural systems could be suited to present day, fantasy or sci fi, however.