The fish is thought to have been chosen by the early Christians for several reasons:
- the Greek word for fish (ICHTUS), works nicely as an acrostic for "Jesus Christ, God’s Son, Savior”.
- The fish would not be an obvious Christian symbol to persecutors
- Jesus' ministry is associated with fish: he chose several fishermen to be his disciples and declared he would make them "fishers of men."
The second fish symbol is the ICHTHUS fish, with the Greek word for fish written out to emphasize the symbolic acrostic described above.
Although the word looks like IXOYE, the letters are from the Greek alphabet, so the "I" is actually an iota, the "X" is actually a chi, the "O" is actually a theta, the "Y" is an upsilon, and the "E" or "C" at the end is a sigma. Taking the first sound from each of these Greek letter names, we get the transliteration into our alphabet of ICHTHUS.
Today, when Christians (in the West) do not need to worry about persecution, the Christian fish symbol often has "Jesus" written inside or includes a cross symbol. And of course, there have been many spoofs and variations of the popular Christian symbol, such as the famous "Darwin fish" (with legs).
The fish is also a symbol of baptism, since a fish is at home in the water.
Patricia S. Klein, Worship without Words: The Signs and Symbols of Our Faith (2000).
Carolle E. Whittenmore, ed., Symbols of the Church.
George Wells Ferguson, Signs & Symbols in Christian Art.
Frederick Rest, Our Christian Symbols.