About 10 years ago in a marriage preparation class, our instructor was a religious historian. This was just before the days of being able to look up anything and everything online in a matter of seconds, so not a lot of people knew the story at the time.
Being a Christian was not cool in the early years of the religion - in fact, being one was reason enough to be devoured by lions in the Colisseum (this was before they had football, hockey, or even lacrosse). Going to Church on Sunday wasn't something you did out of peer pressure; one had to risk one's life to even attempt to find out if someone else was a Christian. So what you did was this: walking along the dusty streets and common areas, you might have come across someone and started chatting. You would then casually draw a meaningless arc in the dirt. If the other person did nothing, you could just scratch it out or keep doodling or whatever; however, if they other person ALSO drew an arc in the dirt, beginning at a point of your arc and then overlapping yours to make a simple fish symbol out of the 2 overlapping arcs, he would clandestinely have just identified himself as a Christian. And you, also being a Christian, would then know it was safe to discuss such things with him. If you weren't a Christian yourself, you wouldn't have thought twice about the other person making a fish symbol out of your arc.
Supposedly the Romans never cracked this simple code, making it one of, if not the, longest uncracked code in history (of course, there may be other uncracked codes that are still secrets and date back to an even earlier time than 100 AD, so if you are a member of one of those secret orders/brotherhoods/societies, feel free to share your secret handshake here with our community. I won't tell ANYONE.)