As some have said, I would attempt to meet the person who wrote the letter. Invite them over for tea. Let them meet me and see than I don't match the description she provided. Then I would ask them where they go to church and tell them about my walk.
After that, I'd ask them why they objected to my NRA stickers. Then I'd ask them if they felt that way, why not put a sign in their yard saying that their home is "gun free"? When they quickly recognize how unsafe that would be, point out that the NRA stickers make you safer.
You can go a long way down the road of civility with common sense before you even start talking about your weapons.