Were it me I would be making a complaint to my local law enforcement agency. This sort of thing seldom stops at one incident.
Your neighbor has several issues here none of which are good. It might be well to remember that people have committed all manner of hideous crimes while hiding behind the "Christianity" banner but it is obvious that this person is neither a Christian nor a man.
I agree with Callmaker, lodge a complaint so it is documented. Unless you comply with their wishes and remove the sticker and get rid of your guns, this probably won't be the only time you will hear about it.
I have dealt with neighbors like this. Ended up actually getting a restraining order against him.
You should absolutely file a complaint with your local PD. Make a copy of the note first before turning it over to them. I also will say that I am a Christian Gun Owner, and I feel that this person is not a true Christian. This note demonstrates Intolerance on many levels as well.
Now what I would do also, get one of those signs that says "My neighbors wish to have all guns removed from this neighborhood. They are Gun Free! I promise NOT to use my guns to protect them in any way!" Anyway you get the idea but again, make sure their lunacy is reported.