The fish's first known use as a Christian religious symbol was sometime within the first three centuries AD.

The Greek word for fish is "ichthys." As early as the first century, Christians made an acrostic from this word: Iesous Christos Theou Yios Soter, (Jesus Christ, Son of God, Savior).

The Greeks, Romans, and many other pagans used the fish symbol before Christians, so it attracted little suspicion, making it a perfect secret symbol for the persecuted believers. When threatened by Romans in the first centuries after Christ, Christians used the fish to mark meeting places or to distinguish friends from foes. According to one ancient story, 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 like-minded company.

The ichthys symbol has been re-adopted as a badge by modern Christians, often with the word "JESUS" in the center of the symbol.