@ehBeth,
The temptation here is to rhetorically blow her head off, but that's just axe grinding. I think ehBeth has the most responsible answer: Ignore her and only talk to her about things related to her employment.
If she has the time to stand in your office's doorway and preach, then she isn't doing the job she is being paid to do. There's nothing wrong with reminding her of that.
If she just wants to loiter around your door, close it.
If she wants to talk through the door, put music on.
If any of that makes her think that you just can't handle the big picture truth of Christianity etc etc etc, so be it. Let her think whatever she likes.
An example:
She says: "Hey Dupre, did you read the flyer I stuck under your door?"
You can...
A) give no reply
B) give a mild reply: "No, and please stop."
C) give a medium reply: "Shouldn't you be cleaning?"
D) give a harsh reply: "You are paid to clean, not litter my office or disturb me while I'm working. One of us is doing the job they are being paid for."
While it doesn't bring any understanding to her as to why her behavior is rude, A is probably the best IMO. The others while more direct, are the kinds of things that (fro your description) would only serve to further fuel this kind of intrusion.
Other suggestions would include getting a lock for your office door and a radio.
If she is insistent enough that you hear her out, make HER PAY YOU for your time. Lastly, if it gets to the point where she is obstructing your work and the harassment becomes more and more intrusive or physical, you will be forced to take this to your mother and force the point. If she isn't willing to deal with it then, she is failing you.
I'm not sure what the circumstances are that you are in her home, but even if she's doing you a favor, you deserve a harassment free home and workplace.
T
K
O