The Sign of the Fish
The Greek word for fish is "ichthus." As early as the first century, Christians used the word to make an acrostic: the letters stood for " Iesous Christos Theou Huios Soter", i.e. "Jesus Christ, Son of God, Savior." The fish has plenty of other theological overtones as well — Christ fed the 5,000 with 2 fishes and 5 loaves, and He called His disciples "fishers of men."
Greeks, Romans, and many other pagans used the fish symbol before Christians. Hence the fish, unlike, say, the cross, attracted little suspicion, making it a perfect secret symbol for persecuted believers. Christianity had been declared illegal and public gatherings of Christians were not allowed. When threatened and persecuted by Romans in the first centuries after Christ, Christians used the fish symbol to mark meeting places and tombs, or to distinguish friends from foes.
According to one ancient story, when a Christian met a stranger in the road, the Christian sometimes drew one arc of the simple fish outline in the dirt. If the stranger drew the other arc, both believers knew they were in good company. Current bumper-sticker and business-card uses of the fish hearken back to this practice.
Certainly this divinely provided reminder speaks volumes about what is at the heart of the Christian's faith. Jesus — He lived and walked among us so He could die for us. Christ — the Messiah that believers in the Only True God had been anticipating coming for 6000 years, the answer to the Jews' hope in the fulfillment of God's promises to Abraham. God's Son — the only begotten Son, sent to earth on a mission, totally God, totally man, and totally committed to accomplishing the Father's mission. Savior — He was made sin for us who were already dead in our sin, died to pay the penalty for our sins, and was resurrected from the grave so that we could be redeemed to a new eternal life.
Critics of the fish symbol either decry it as tacky tokenism or point out that the fish still carries baggage from the days when pagans used it to represent fertility or, more specifically, the female reproductive organs. Though I agree that ichthys symbols in phone-book ads seem to commercialize faith, I don't find the pagan argument compelling. No symbol means the same thing to all people at all times. That early Christians succeeded in transforming an already powerful symbol proves their interpretive creativity, not their ignorance or a tendency to syncretism.