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" (ichthys) as an anagram/acronym for "Jesus Christ God's Son, Savior."
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.'
As the the Christian Church grew, 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.