I dug this up from an old thread. Victor08 posted it.
For your consideration. http://www.eureka4you.com/fish/fishsymbol.htm
The History of the Christian Fish Symbol
The fish's first known use as a Christian religious symbol was sometime within the first three centuries AD. Christians began using the Greek word for "fish" as an anagram/acronym for "Jesus Christ God's Son, Savior." More about this later.
The fish outline is a logical symbol for the early Christian church to adopt. Not only was fish a common food of the day, it was also used by Jesus during His ministry.
A 'fishy' tale?
In the years following the ascension of the resurrected Jesus to heaven, the Christian church grew rapidly.
Christians soon found themselves to be the subjects of persecution by both the Romans and the Jews.
In many locales, it became dangerous to be known as a Christian.
Thus, when two strangers met and thought maybe they were fellow believers, one of them would draw, on the ground, the upper half of the fish symbol.
Recognizing the symbol, the stranger would add a second curved line and complete the drawing of a fish.
It is a very simple shape to draw - just two curved strokes. It could be drawn quickly, and erased just as quickly if there was no sign of recognition on the part of the stranger.
The 'Greek' connection, #1
We do not know whether the story above is true but we do know that the fish's first known use as a Christian religious symbol was sometime within the first three centuries AD. Possibly around the 16th century Christians began using the Greek word ichthys for "fish". Ichthys is the most commonly used word in the New Testament for fish.
Ichthys consists of five letters from the Greek alphabet: I-ch-th-y-s. When these five letters are used as initials for five words, we obtain this Christian Declaration: Iesous Christos Theou Yios Soter. This is an acrostic for 'Jesus Christ, God's Son, Savior.'
IXQUS ----- Ichthys ----- Jesus Christ, God's Son, Savior
I Iota the first letter in the Greek word Iesous Jesus
X Chi the first letter in the Greek word Christos Christ
Q Theta the first letter in the Greek word Theos Of God
U Upsilon the first letter in the Greek word Yios (Huois) Son
S Sigma the first letter in the Greek word Soter Savior
Greek meaning Iesous Christos Theos Yios Soter
English transliteration ... Jesus Christ Of God Son Savior