The first Gospel to be written opens with this announcement:

The beginning of the good news about Jesus the Messiah, the Son of God …
ἀρχὴ τοῦ εὐαγγελίου ἰησοῦ χριστοῦ υἱοῦ θεοῦ … (Mark 1:1)

And in Matthew’s Gospel we are told why he was named Jesus (which means “he saves”):

… you are to give him the name Jesus, because he will save his people from their sins.
καλέσεις τὸ ὄνομα αὐτοῦ Ἰησοῦν· αὐτὸς γὰρ σώσει τὸν λαὸν αὐτοῦ ἀπὸ τῶν ἁμαρτιῶν αὐτῶν (Matthew 1:21)

‘Son of God’ and ‘Saviour of the world’ were titles claimed by Augustus Caesar (as referenced, for example in the Calendar Inscription of Priene), who was the Roman Emperor at the time of Jesus’ birth. The followers of Jesus boldly proclaimed that it was not Caesar but Jesus Christ who is Lord, the true Son of God and Saviour of the world.

The initial letters of the words in that simple credal statement in Greek (the common language of the Roman Empire) intriguingly spelt out the word for ‘fish,’ which thus became an early symbol of allegiance to Christ.

Some further realised that the letters of the Greek word for ‘fish’ could all be found in a single disc cut into 8 segments like a loaf of flat bread.

Which calls to mind Jesus’ miraculous feeding of both the 5000 and the 4000 with loaves and fish.

And I wonder whether the initial letters of the shortened declaration that “Christ is Lord” may have been what first inspired the form of the ‘fish’ symbol.