The ichthys is seen in 1st-century catacombs in Rome. According to tradition, ancient Christians, during their persecution by the Roman Empire in the first few centuries after Christ, used the fish symbol to mark meeting places and tombs, or to distinguish friends from foes:
…when a Christian met a stranger in the road, the Christian sometimes drew one arc of the simple fish outline in the dirt. If the stranger drew the other arc, both believers knew they were in good company. Current bumper-sticker and business-card uses of the fish hearken back to this practice. The symbol is still used today to show that the bearer is a practicing Christian. —Christianity Today, Elesha Coffman, "Ask the Editors"
There are several other hypotheses as to why the fish was chosen. Some sources indicate that the earliest literary references came from the recommendation of Clement of Alexandria to his readers (Paedagogus, III, xi) to engrave their seals with the dove or fish. However, it can be inferred from Roman monumental sources such as the Capella Greca and the Sacrament Chapels of the catacomb of St. Callistus that the fish symbol was known to Christians much earlier. Another probable explanation is that it is a reference to the scripture in which Jesus miraculously feeds 5,000 people with fish and bread (Matthew 14:15-21, Mark 6:30-44, Luke 9:12-17, and John 6:4-13). The ichthys may also relate to Jesus or his disciples as "fishers of men" (e.g., Mark 1:17). Tertullian, in his treatise On Baptism, makes a pun on the word, writing that "we, little fishes, after the example of our ΙΧΘΥΣ Jesus Christ, are born in water." Still another explanation could be the reference to The Sign of Jonah. Just like he was in the belly of a big fish, so Christ was crucified, entombed for three days, and then rose from the dead.
