Did Christians ever come up with "secret" ways of knowing who other Christians were? I don't mean exactly a handshake, because I get that sometimes people don't (or can't) shake hands. Seems like it might be a bit pointless, like a joke you have with friends or something. But did they?
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The Sator square was likely used to secretly advertise a house as Christian to other Christians in the first century.
The idea of the ICTHUS (or Christian fish) symbol was an early example of this. It wasn't pointless, as it was used to identify each other in a society where persecution of Christians was extremely common. One person would draw one arc of the fish, probably in the dirt, and another would draw the other if they were a Christian as well. This wasn't done just for fun, but for protection, in much the same way as how WWII Allied soldiers would whisper "flash!", and another Allied soldier would respond with "thunder!" in order to identify themselves.