Acrostics are wordplay with a job. This one became a sign you could scratch without writing the theology out in full. The sentence is a confession. The animal is what the first letters do when you stack them.
It sits in the wordplay drawer, and it is also why the journal’s Christianity category is not sealed off from the games. A creed can scan, and it can hide a fish.
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Ἰησοῦς Χριστὸς Θεοῦ Υἱὸς Σωτήρ — Jesus Christ, Son of God, Saviour. The initials spell ἰχθύς, fish. That is why a fish could stand in for the whole sentence: the picture is an acronym you can draw.