I recently posted the following in the comments on this post of Rabbi Gil Student, which is about how to (or not to) interact with missionaries. As is my wont, I'm reposting it here. I have other thoughts on this matter, but I'll make them into a separate post at some point.
I once had a very pleasant conversation (mostly about animals; not at all about religion) with a Utahan lady at Yellowstone who had lent me her family's binoculars so I could look at some mountain goats. At the end of the conversation she rather apologetically (and, I might add, nervously) explained that she knew (from my kippah) that I was a Jew; she was a Mormon, and asked me whether I would accept her pamphlet. I took it - quite graciously, I think - and she thanked me with obvious relief and genuine appreciation, shaking my hand. (I later threw the pamphlet away, several states' distance out of her sight.) I believe (hope) my actions were a kiddush hashem. (Maybe I shouldn't have shaken her hand.)
My point: many (probably most) missionaries have undertaken an often unpleasant, thankless task in order to convince people of something that they themselves honestly believe, in an effort to save those others from a terrible fate and/or to serve and glorify God (all values we hold). We obviously don't agree with their understanding of theological fact, but given their beliefs, they're doing a noble thing, and for that I can't help but respect them. One can treat them with respect without implying that one agrees with their religious views. I think that's the way to go.
Who knows? Some missionary may be so impressed by how Jews treat him that he's won over to our beliefs.