The ichthys, two intersecting arcs forming the outline of a fish, is among the earliest Christian symbols, in use by the second century and well before the cross became a common devotional sign. Its currency rests on a Greek acrostic: the letters of ichthys, fish, were read as Iesous Christos Theou Yios Soter, Jesus Christ, God's Son, Saviour. Because the confession is folded inside an ordinary word, the sign could circulate discreetly among Christians in the period before Constantine, though the familiar story that it served as a secret recognition token, one believer scratching an arc in the dust and a stranger completing it to prove their faith, is a later embellishment rather than documented ancient practice. The two clearest early attestations are the epitaph of Abercius, bishop of Hierapolis in Phrygia, composed before about 216 and rich in fish and eucharistic imagery, and the Licinia Amias inscription from Rome, of similar date, which pairs fish and anchor without naming Christ outright, evidence that the symbol was already doing devotional work that plain text could not risk doing openly.
A more elaborate variant, sometimes called the ichthys wheel, superimposes the five Greek letters radially into an eight-spoked figure; its best-known example, a marble slab from Ephesus, is widely presented as covert persecution-era signage, but the slab is generally dated to the Byzantine period, well after toleration, so it attests the acrostic's long afterlife rather than any early concealed use.