Since the days that Christians worshipped in the catacombs to avoid persecution, the fish has been a symbol of Christianity, just as the star of David is a symbol of Judaism and the crescent symbolizes Islam.  Visitors to the Calixitus catacombs outside of Rome, see the symbol carved in the limestone walls where Christians once met and worshipped.  Many throughout the world still display the fish—whether it is lapel pin or a small insignia on the bumper of a car—to indicate their faith as Christians.