In the times of the early Roman Christians, followers of Christ were often persecuted for their faith and any outward symbol of their belief could often get them imprisoned or killed. However, for their underground movement, the practitioners needed some means of identifying one another, and for safety they chose a symbol that was not directly associated with their faith. Thus the fish symbol was born¹.

For the symbol to be a success there had to be some recognisable links with the faith. The fish was said to symbolise the baptismal waters, the loaves and fishes that Jesus used to feed the five thousand, and Jesus's own declaration to the disciples to become 'fishers of men'. The Greek word for fish, ichthus, is an acrostic² consisting of the initial letters of five more Greek words, which briefly but clearly described the character of Christ and His claim to the worship of believers: 'Iesous Christos Theou Yios Soter' which translates as 'Jesus Christ, Son of God, Saviour'.

The symbol of the fish and the word ichthus gained a meaning of high significance for those early Christians, and is still recognized as a Christian symbol today. Chromed fishes can often be seen stuck onto the back of Christians' cars, and for some time a chalk fish-mark was used by homeless wanderers to mark the houses of people who could be depended on for charity or for food.

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¹ The use of the fish is documented in the movie Quo Vadis, a film focusing on the persecution of the Christians under the Roman Emperor Nero.
² A puzzle in which certain letters in each line form a word or words.