Nice Young Man: Hello ma'am. I'm XXX and this is XXX. What a lovely house you have? Have you lived here long? It looks like you're just moving in.
Me: We moved in a couple months ago but we're still finishing some renovations.
NYM: We're new to the area as well.
Me: So you guys are on your mission in DC.
NYM: (Looking a little surprised). Why, yes.
Me: I've had Morman friends before so I know a bit about your faith.
NYM: Well we're out here today to spread the message about Jesus Christ. Do you have a few minutes to talk.
Me: Thanks but I'm actually not religious, so I'm probably going to be a tough sell for you guys. But there are lots of nice people in this neighborhood who you might find to talk to.
NYM: So do you mean that you don't believe in god at all or you just don't belong to a particular church?
Me: I'm an atheist. But I'm glad we live in a county where we both can believe whatever we want and still all get along.
NYM: Well take this card in case you ever have any questions about the meaning and purpose of live.
Me: Thanks. Good luck and have a nice day.
NYM: You too!
It was actually a very pleasant exchange with lots of smiles. Even though I clearly don't agree with their beliefs, I can appreciate that it must be hard for barely adult Mormons to go door to door trying to talk about their religion with strangers. I'm sure they get lots of rude responses and doors slammed in their face (even from other Christians). And regardless one's own views, I think you should always be cordial and polite.
I also hope they walked away with a better opinion about atheists. How ironic if one of the friendliest faces they encountered today was someone who doesn't even believe in god?