The Fish Symbol or ICHTHYS

       The ichthys or ichthus (/ˈɪkθəs/), from the Greek ikhthýs (ἰχθύς 1st cent. AD Koine Greek [ikʰˈtʰys], "fish") is a symbol consisting of two intersecting arcs, the ends of the right side extending beyond the meeting point so as to resemble the profile of a fish. Now known colloquially as the "sign of the fish" or the "Jesus fish". 

       According to tradition, ancient Christians, during their persecution by the Roman Empire in the first few centuries after Christ, used the fish symbol to mark meeting places and tombs, or to distinguish friends from foes:

"According to one ancient story, when a Christian met a stranger in the road, the Christian sometimes drew one arc of the simple fish outline in the dirt. If the stranger drew the other arc, both believers knew they were in good company. Current bumper-sticker and business-card uses of the fish hearken back to this practice."

— Christianity Today, Elesha Coffman, "Ask The Expert"

       There are several other hypotheses as to why the fish was chosen. Some sources indicate that the earliest literary references came from the recommendation of Clement of Alexandria to his readers (Paedagogus, III, xi) to engrave their seals with the dove or fish. However, it can be inferred from Roman monumental sources such as the Cappella Greca and the Sacrament Chapels of the catacomb of St. Callistus that the fish symbol was known to Christians much earlier. Another probable explanation is that it is a reference to the scripture in which Jesus miraculously feeds 5,000 people with fish and bread Matthew 14:15-21, Mark 6:30-44, Luke 9:12-17, and John 6:4-13). The ichthys may also relate to Jesus or his disciples as "fishers of men" (e.g., Mark 1:17). Tertullian, in his treatise On Baptism, makes a pun on the word, writing that "we, little fishes, after the example of our ΙΧΘΥΣ Jesus Christ, are born in water." Still another explanation could be the reference to the sign of Jonah. Just like he was in the belly of a big fish, so Christ was crucified, entombed for three days, and then rose from the dead.

The ICHTHYS with lettering.

ΙΧΘΥΣ, or also ΙΧΘΥϹ with lunate sigma (Ichthys) is a backronym/acrostic for "Ἰησοῦς Χριστός, Θεοῦ Υἱός, Σωτήρ", (Iēsous Christos, Theou Yios, Sōtēr) contemporary Koine [ie̝ˈsus kʰrisˈtos tʰeˈu (h)yˈjos soˈte̝r], which translates into English as "Jesus Christ, Son of God, Saviour."

• Iota (i) is the first letter of Iēsous (Ἰησοῦς), Greek for "Jesus".
• Chi (ch) is the first letter of Christos (Χριστός), Greek for "anointed."
• Theta (th) is the first letter of Theou (Θεοῦ), Greek for "God's", the genitive case of Θεóς, Theos," Greek for "God."
• Upsilon (y) is the first letter of (h)yios (Υἱός), Greek for "Son".
• Sigma (s) is the first letter of sōtēr (Σωτήρ), Greek for "Savior."

       This explanation is given among others by Augustine in his Civitate Dei, where he notes that the generating sentence " Ἰησοῦς Χριστὸς [sic] Θεοῦ Υἱὸς Σωτήρ" has 27 letters, i.e. 3 x 3 x 3, which in that age indicated power. (This suggestion is obviously spurious, resulting from Augustine's ignorance of Greek.) Augustine quotes also an ancient text from the Sibylline oracles whose verses are an acrostic of the generating sentence.
       A fourth century A.D. adaptation of ichthys as a wheel contains the letters ΙΧΘΥΣ superimposed such that the result resembles an eight-spoked wheel.

Scripture References for ICHTHYS Chrismons:

• "Jesus called out to them, "Come, follow me, and I will show you how to fish for people!" Mark 1:17 (NLT)
• "Then some of the scribes and Pharisees answered him, saying "Teacher, we wish to see a sign from You." And answering He said to them, "An evil and adulterous generation seeks for a sign, and no sign will be given to it, except the sign of Jonah the prophet. For just as Jonah was in the belly of the great fish three days and three nights, so the Son of Man will be in the heart of the earth three days and three nights. The men of Nineveh will stand up in the judgment with this generation and will condemn it." Matthew 12: 38-42 (BLB)
• "You killed the author of life, but God raised him from the dead. We are witnesses of this." Acts 3:15 (NIV)