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, i.e. Jesus Christ, Son of God, Savior.

The earliest uses of the Ichthys (Greek for "fish") symbol date back to the 2nd and 3rd centuries CE, where it appeared in graffiti, inscriptions, and funerary epitaphs. Rather than a secret code ...

you could be arrested or even killed for practicing your religion. So some Christians marked locations where they would meet to celebrate and pray with the ichthys. It's like The DaVinci Code ...