Nykaela wrote:
But what if you wrote a story set in the Americas? Not even a thought about God, or would it work to slip some mention in there? What about Asia?
Two things come to mind in answer to this question.  
One is this as early as 200 AD we have evidence that Christians came to the Americas, not certainly, but possibly (and way earlier for Asia). Thus, in the case of the Americas, there really is no time in known history that Christianity couldn't be brought into the story somehow because anything prior to about 200 AD is mostly myth in terms of what we know.  This isn't so much the case for Asia though which leads me to my second point.
As pastor, author, and theologian points out often a son can learn what kind of father he needs to be through the negative example of his father. You see the very fact that a son knows his father is a bad father means that he must know what a good father is. This can also be applied to history. You don't only have to portray what is right through doing right. You can also portray it through wrong as long as you make it very clear that it is evil.  
If I where to do a story, say in early American history, I would do a story about people today looking back to that day somehow or another, and end the story with a comparison of the hope that we have in Christ as opposed to the despair that they suffered under nations like the Aztecs.