All right, we are moving into the realm of esoterics. The question is what is the point of historical fiction? To answer this question we need to look at the three aspects of a novel: storyline, plot, and theme. Obviously, the storyline has to convey some degree of adventure or mystery--entertainment, remember entertain your readers. In a historical fiction novel, the storyline must convey the entertaining aspects of the life or portion of the life of your characters in a way that is entertaining to your readers. Therefore, the storyline must be steeped in the times. This is where the true voice of the times and culture come out. This is where you must be completely true to the historical times and place. The storyline may be contrived, but not the setting and thinking of your characters. The historical view is contained within the storyline. So what about the plot? The plot is the pattern of events that make up the larger narrative. In the plot, you must also hold to the basic historical view of the times that is conveyed in the storyline, but there is a larger point in it. The storyline must be made up of real elements, elements that are not out of place in the times of your novel. The plot may not stray from historical accuracy, but the pattern of events may not be from history at all. You might follow a character little known in the history of the world or who is a complete fabrication. The events of the storyline are real in terms of the past, the plot may deviate, not in history, but in the overall pattern of events, because, ultimately, your theme will hopefully not be the same as any other novel. In fact, your theme will hopefully be unique to your time and views. That is the point of the theme and that is the point of a historical novel. You convey to your readers a theme that is wholly yours and wholly your time, but that is wrapped perfectly in a historical plot and storyline.
Let's be very clear, your theme cannot be a convenient fiction to get across your personal message. For example, historical fiction is likely not the best means to get across an anti-religious theme. To do so would compromise the storyline and plot. Historical fiction is likely not the best way to get across many modern ideas--they are too foreign to the past. Many themes have been plumbed a little too much in historical fiction, and some are just trite. For example, an anti-slavery theme would be great for the 1800s but a little overwrought today. An anti-patriarchal theme would fit in the early 1900s, but not so much in the 21st century. A closer novel to today would better convey these themes. The most powerful themes for historical fiction are comparative themes. Those that draw comparisons from the past and lure the reader into the past to then spit them back up into the real world with a piece of the golden fleece or a golden apple in their hands. The greater the differences and the greater the comparisons, the more powerfully the theme can draw and affect your readers. So, examples...
Dana-ana http://www.goddessnovel.com/ is a historical fiction themed novel in a modern setting--it likely would not count as historical fiction. It pulls a character from the past into the modern world and allows the readers to see, by comparison, the strengths and weaknesses of the Anglo-Saxon culture. Especially, the religious fervor of Dana-ana becomes apparent. The direct comparison is to the view of religion in the modern world. The difference is the way Dana-ana goes about her experience of life, love, and religion. The theme is about redemption.
Aegypt http://www.aegyptnovel.com/ is a historical fiction novel in a historical setting--1926. Its theme asks ultimate questions about what might be true. It puts up four competing arguments. First, is truth only in what we believe? Second, is truth only in what we perceive? Third, is truth only in what we know? Fourth, is truth only in what we can understand? This is the theme, the storyline and the plot are very different because it is not a philosophical treatise, it is a fiction novel. The point of the novel is to use historical comparisons to bring up and answer these questions in the mind of the reader.
Centurion http://www.centurionnovel.com/ is a historical fiction novel that follows the life of a known historical person, the Centurion who executed Christ. It posits a very historically based theme about the difference between belief and truth. The point it asks of the reader is why would the Centurion at the foot of the cross say, "Surely this was the son of God." This is the theme of the novel.
So in historical fiction, the theme may be historically based, or not. The storyline and plot must be.