Explanation: In Christian mythology, the ichthys symbol, depicted, was used as a secret sign of one’s faith. To outsiders, it would appear only as an innocent fish - to those in the know, it would be a sign of belonging to the DANGEROUS CULT OF CHRISTIANITY.

It still sees use today, though not as a secret so much as virtue signaling.

The reason for them choosing the symbol was because icthys was an anagram:

Ιησους - Jesus Χριστός - Christ Θεού - of God Υιός - son Σωτήρ - saviour
I.e. Jesus Christ, son of God, (our) saviour
Or maybe they just thought God was a fish ¯\_(ツ)_/¯

Abbreviation maybe? An anagram would be scrambling the letters to remake a phrase with the same letters in a different order