The fish symbol was used as a code to escape persecution. Early Christians used the fish to identify themselves to other Christians. The Greek word for "fish" (IX0YE) contains the initals of the names and titles of Christ.
I Iasous = Jesus
X Christos = Christ
O Theou = God
Y Uios = Son
E Soter = Savior
When two people met, one would draw the upper arc in the sand and the other person could identify himself as a Christian also by drawing the bottom arc, completing the sign of the fish.
