The entire point of using a symbol is that it conveys meaning and saves space -- you see one picture of a stick figure in a dress and you no longer need the phrase "This is the place where female humans can discharge waste." But what is fascinating is that sometimes the meaning of a symbol will get lost to history, but we'll just keep right on using it anyway.
But would we do that if we knew ...
Note: Did you know that Cracked used to be a magazine? Of course you didn't, none now live who remember that. But it's true -- that goofy little C you see at the top of your tab is a symbol that now means: "you are on the internet, reading comedy jokes, for free." But it used to mean: "you are reading a magazine, and if you don't buy it soon, the guy behind the counter is going to start giving you shit." See? The world's a madhouse! Constantly changing and defying any hope for any of us to understand it! And in this Cracked Classic, it gets even worse! -Cracked
Apart from the cross, the most ubiquitous symbol of Christianity is the ichthys, known to us as the Jesus Fish, and today it appears predominantly in its natural habitat -- car bumpers. The ichthys actually dates right back to ancient times, when Christianity was still an obscure sect, and considering that fish and fishing were frequently used as symbols in the Bible, you could argue that it's a more appropriate symbol for the teachings of Christ than the device used to torture and kill him.
"Seriously, guys? Do you wear tiny rifle necklaces to remember Martin Luther King Jr.?"
It's a vagina.
One of the names given to the pre-Jesus Jesus Fish is the vesica pisces (vessel of the fish), and it was used as a symbol of every female fertility god ever, from Atargatis (the Syrian fertility goddess), Aphrodite/Venus (the goddess of love and sex) to the pagan Great Mother goddess, where it symbolized her life-giving vulva. Basically, whenever you encountered an image of fish in the pre-Christian world, it was probably an opposite-of-subtle metaphor for lady parts.
According to some researchers, Christians adopted the vagina-fish symbol simply because of how common it was, but later looked for all sorts of non-vaginal justifications for it. Even actual Christian scholars admit that their second most popular symbol has a colorful history, just not one you want to bring up during a family dinner party.