The Fish
In its early years, Christianity was an illegal religion. Because of this, the "Sign of the Fish" became a universal symbol that secretly identified believers to one another. The Greek word for "fish" is IXOYS. These five letters were also the first letter (in Greek) of the words JESUS CHRIST, GOD’S SON SAVIOUR. By drawing the "Sign of the Fish", Christians revealed both their identity and the basis of their faith.
About.com