I know in recent years the Christian fish symbol has fallen into some pretty tacky and over used marketing (much like the ruin of the Christian holidays, Christmas and Easter). However, it seems to have had a noble beginning, one with which I am still glad to associate myself. It helps me recall the still dangerous situations many of my persecuted Christian brothers and sisters find themselves around the world today.
In the early years of the church a famous acrostic was made in Greek for the acclamation “Jesus Christ , God’s Son, Savior” (Iesous Christos, Theou Uios, Soter). In Greek ch is X and th is Θ, so the initial letters of this phrase spelled IXΘUS. U is interchangeable with y so IXΘUS could also be IXΘYS, which just so happens to be the Greek word for fish, thus the frequent use of the fish by early Christians and up to now as a symbol for Jesus Christ.
Greeks, Romans, and many other pagans used the fish symbol before Christians. So, unlike the more obvious cross symbol, the fish attracted a lot less attention which made it the perfect secret symbol for persecuted believers. When threatened 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 might draw one arc of the simple fish outline in the dirt; if the stranger drew the other arc, both believers knew they were in trusted company.