PrincessoftheKing wrote:
 So far, in my book, I haven't had any religion whatsoever. The worldview of the book is Christian, but the world itself doesn't have any religion. In other words, good and evil is laid out very clearly, and the main characters model Christ-like attributes (or the opposite, when it comes to the bad guys. 

 ), but there is no defined God.
Is that bad? Do I need to have God in my stories, like He is in the real world? Does not having Him make my story - I don't know - atheistic? 
Yes, there needs to be One. Without God, some "other" Authority, you can have no morality.
EDIT: Allow me to clarify - no 
objective morality. Everything is up for grabs.
PrincessoftheKing wrote:
 And, if I have Christianity in my story, do I need to develop the history of the world to match ours, at least a little? I write with the assumption that the world was created, and then fell, and my story lies somewhere after the fall. But do I need to say that a savior has either come already or will come sometime in the future? Or can I leave all that undefined? 
At least a little, yes. 
As for the Savior already having come or not, that would depend. The former, Him already come, would be better, methinks, as it would be more of a New Testament versus an Old Testament atmosphere (while salvation through faith has, of course, always existed - Romans 4, wasn't it? - things were, of course, slightly different in the Old - e.g., animal sacrifices).
PrincessoftheKing wrote:
 Then... I feel like, if I have the True God, there should also be false gods. If there weren't, the world would be extremely unrealistic. 
It depends on what you mean by "false gods." False gods are described in the Bible, but they're not actually real, in the sense of being some "god."
PrincessoftheKing wrote:
 If I have all that, then I think it would have to become a somewhat major plot point; after all, our whole purpose in life to glorify God, and to tell others about Him... so it wouldn't be any different in my story. 
Yep.
PrincessoftheKing wrote:
 But I don't want to be preachy, and I want my book to appeal to a non-Christian audience, so that I can reach unbelievers with my writing. 
Why (to both statements)?
God bless,
Joel ><>.