ΙΧΘΥΣ

The fish was an early symbol of Christianity that endures today on bumper stickers and businesses as a sign of Christian faith. The fish is thought to have been chosen by the early Christians for several reasons: the Greek word for fish (IXOYE), works nicely as an acrostic for "Jesus Christ, God's Son, Savior," it would not be an obvious Christian symbol to persecutors, and Jesus' ministry is associated with fish in that he chose several fishermen to be his disciples and declared he would make them "fishers of men." It is said that during the persecution of the early church, a Christian meeting someone new would draw a single arc in the sand. If the other person was a Christian, he or she would complete the drawing of a fish with a second arc. If the second person was not a Christian, the ambiguity of the drawing would not reveal the first person as a Christian.

ReligionFacts.com. "Christian Symbols A to Z". ReligionFacts.com.
    2004. http://www.religionfacts.com/christianity/symbols/a-z.htm