A fish symbol, Ichthus, proclaims a Christian. Modern car bumpers abound with them. They mean "Christian here" and so they did for the early Christians, who used the symbol to identify each other. Ichthus is Greek for fish and the early Christians made an acronym (the first letter of several words) to spell out ICHTHUS. Now the sign of the fish is so popular and recognised that Christianity's critics parody it, says Christianity Today editor-at-large Collin Hansen.
The acronym ICHTHUS means "Jesus Christ, Son of God, Savior" and it breaks down thus, says Del Birkey, writing in the Church House Book website: Iota (i) is the first letter of Iesous (Greek for Jesus); Chi (kh) is the first letter of Khristos (Greek for Christ); Theta (th) is the first letter of Theou (Greek for God or God's); Upsilon (u) is the first letter of Huios (Greek for son); Sigma (s) is the first letter of Soter (Greek for Saviour). Thus the letters Iota+Chi+Theta+Upsilon+Sigma often appear inside the fish symbol, as something resembling "IXOYE" but in Greek letters.
Hansen, in an article for Christianity Today, recognises Ichthus as an enduring symbol of Christianity, but reminds us it had pagan origins. Greeks and Romans joined the outlines of two crescent moons to make the same symbol, but depicting "feminine fertility and deity," he says.
Hansen agrees with Dirkey that Christians first used Ichthus as an acronym, but he reminds us of the deeper significance of fish to Christians. In Mark 1:17, Jesus tells his disciples he will make them "fishers of men," a role that Hansen notes "evangelistically inclined Christians of all ages" adopted as their own. Jesus works miracles, feeding multitudes with a few fish and overloading Peter's fishing boat with a huge catch. Hansen says the early church favoured the fish theme for its many applications to Jesus' life and teaching.
As persecution became more common for Christians throughout the Roman Empire, Hansen says the fish symbol "became a password shared by underground believers." He says it marked secret meeting places and Christian gravestones, even the homes of believers.
Christians used the symbol to test strangers they met to see if they were fellow believers, says Hansen. Meeting on the road, one would draw the first half of the arc and if the other completed the symbol, they knew each other to be Christians without alerting the Roman authorities. And since it was already a pagan symbol, Christians were safer using it than the cross to announce their calling, says Hansen.