The fish, and the acrostic inside it

Greek for fish is ΙΧΘΥΣ, and read downward the five letters spell Ἰησοῦς Χριστός Θεοῦ Υἱός Σωτήρ — Jesus Christ, Son of God, Saviour. The earliest datable witnesses to the fish as a Christian sign all fall around 180-200: the epitaph of Abercius of Hierapolis in Phrygia, which calls Christ the fish from the spring whom the pure virgin caught; Clement of Alexandria listing a fish among the devices a Christian may cut into a signet ring; and Tertullian, who turns it into an argument about baptism — we, little fishes, after the example of our ΙΧΘΥΣ Jesus Christ, are born in water. Carvings of the sign are shown to visitors at Ephesus, including a wheel superimposing all five letters, but nothing dates them and no ancient source connects the symbol's beginning with that city or with Timothy.

On Baptism (De baptismo) · On Baptism, ch. I