Sometime during the 1st century the early Christians in the Rome catacombs began using fish symbols for secrecy purposes.

The earliest reference to this symbol was in Paedogogus, III, xi by Clement of Alexandria. In the Greek language, which was commonly used at that time, the word for fish was ichthus (often spelled icthus) or (ixthus).

At some point a creative person or group formed an acrostic using the letters of i ch t hu s. The table below shows the correlation of the word to the acrostic, and its English meaning.