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.
Mark 1:17 "Come after Me, and I will make you become fishers of men."
Matthew 12:40 "...Jonah was three days and three nights in the belly of the great fish, so will the Son of Man be three days and three nights in the heart of the earth."
Matthew 14:17 "And they said to Him, 'We have here only five loaves and two fish.'"
Luke 5:6 "And when they had done this, they caught a great number of fish, and their net was breaking."
Luke 24:42 "So they gave Him a piece of a broiled fish and some honeycomb."
John 21:6 "And He said to them, 'Cast the net on the right side of the boat, and you will find some.' So they cast, and now they were not able to draw it in because of the multitude of fish."
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.