The Christian pacifist & anarchist sets seem to like the Chi-Rho (☧) symbol a lot. But as far as I can tell, that symbol was created as a military standard by Constantine. In a few “I finally got my Chi-Rho tattoo!” blog posts, people reference this line: “Early Christians, through fear of persecution, devised the secret symbol as a means of recognition.”
That comes from the Chi-Rho wikipedia article. But it seems like someone just slipped it in there and there is no reference. I know that the Ichthys (“Jesus fish”) served that purpose. But everything I can find about Chi-Rho says it started with Constantine. Is it just that the minivan crowd co-opted the fish, making it unacceptable for anarchist forearms, and so another symbol had to be enlisted?