Central Idea - The main point of a book. You would find these most likely or mainly in informational texts. These are not found in novels or fictional books. You would find these in fact based books.
Message - (Also known as theme) Message is the main moral of point of the story.The theme is the big idea.
Theme is found in a fictional book or novel, not in a fact based book. Central ideas are found in fact based books.
The main reason why Gene changes his mind about enlisting in the military after he tells Brinker he will is because "<span>D)Gene doesn't want to leave Devon because Finny has returned" since they are quite attached. </span>