Comment: I'd try to defuse the situation

Retaliation will only escalate matters and assure the author feels justified in their actions.

But signing "Christian neighbor" to maintain anonymity makes talking to the person a little more challenging.

Therefore I'd probably make a non-confrontational visit to each of my immediate neighbors homes and ask if they wrote the note or know who did. Of course everyone will deny writing the note but it might help to identify who it was by their lying. In either case I'd ask if they were offended by my sticker. I'd make it clear I'm only there to listen and learn. If given the chance to explain my side great, otherwise no big deal.

At that point if I wasn't satisfied I found the person who did it I might keep a small sign covering my NRA sticker that read "LET'S TALK", but only while parked and only for a week or two if nobody comes forward. At least we'd both know who took the high road.