ΙΧΘΥΣ is Greek for fish. It is an acronym for
Ίησοῦς Jesus
Χριστός Christ
Θεοῦ God’s
Υἱός Son
Σωτήρ Saviour
(Iēsous Christos, Theou Huios, Sōtēr)
The letters can be combined to form a spoked wheel, as in this image at Ephesus:
In the days that Christianity was illegal, and you wanted to check if someone might be a Christian, you could take your staff and draw a semicircle in the dust. If the other person drew another semicircle, completing the simple fish – you had probably safely met another Christian.
There are also parodies of the symbol. A lot are working through the (IMO tiresome) Creationist-Evolution “debate”. With variations on which appears to be triumphing. Or the possibility of combining being a Christian and accepting evolution.
Thanks to my friend who suggested that, from time to time, I introduce a sign or symbol or something similar, on this site.
