The sign of the fish
What is referred to as the Ichthus is a fish symbol (see above) Christians often display on their cars. They do this to let others know they believe in Jesus. Few, however, consider how it begun to represent faith in Christ and fewer still the traditions behind its use.
Though its origin is somewhat uncertain, some believe the association of fish and Christianity began with the Greek word ichthus (or ichthys), translated twenty times in the New Testament as 'fish.' This association was likely encouraged by the fact that at least half of Jesus' twelve disciples were fishermen. Those who pursued this trade included James and John, Peter and Andrew, as well as Thomas and Nathanael (Matthew 4:21 - 22, John 21:1 - 3, etc.). Five out of the first six disciples called by the Lord were fishermen. It was Christ Himself who told Peter and Andrew that instead of catching fish (ichthus) He would train them to "catch" men (an allusion to the preaching of the gospel and the conversions it would bring, see Matthew 4:19, 13:47 - 48, Mark 1:17).
According to several Bible commentaries, some in the early New Testament church used the Ichthus as a secret reference (cipher) for Jesus. This correlation apparently led to the creation of an easy-to-draw shape (see above) as a profession of faith. Catholics state they believe Clement of Alexandria (c. 150 - 215 A.D.) made the earliest reference to a fish-like symbol (1913 Catholic Encyclopedia).
The religious correlation between this image and faith, albeit pagan, pre-dates the birth of Jesus. The pagan deity Dagon (from the Hebrew word for 'fish god,' Strong's #H1712) was one of the gods of Assyria and Babylon. The Amorites (some of which lived just east of the Dead Sea) and the Philistines (Judges 16:23, 1Samuel 5, 1Chronicles 10:10) also worshipped Dagon, who represented fertility. Anciently, he appeared either as a half-man half-fish or as a man wearing a fish-shaped head (with the mouth open and facing up) with its body running down his back.
Some have thought the 'star' (which was actually an angel) which appeared to the Magi seeking the Christ child was a bright object in the shape of a fish, formed by the conjunction of some planets. After the New Testament church started on Pentecost in 30 A.D., it did not take long for violence to visit those who dared to live ‘the Way’ (Acts 9:2). The first martyr, Stephen, died in 32 A.D., just two years after Jesus' resurrection. The Roman Empire's first of TEN state-sponsored hunts and persecutions of those who were converted began under Nero in 67 A.D.
According to some traditions (not the Bible), early believers used the Ichthus to mark off meeting places, to designate tombs, and to identify each other. When a Christian approached someone on the road, he or she (as the story goes) would draw a half-arc on the ground with his or her toe or a stick. If the other person also drew a half-arc, completing the outline of the symbol, then both knew the other believed in Jesus and thus persecution could be avoided.
For whatever reason, the Ichthus or 'Jesus Fish' fell out of popular use for many years until the early 1970s. It experienced a resurgence in use beginning around 1973 and has since become a worldwide icon of the Christian faith. Today, this symbol is not only on cars but also within business logos, advertisements, on T-shirts, in jewelry and more.