ICHThYS is an ancient "password" for the early Christians. When the early Christians couldn't worship publicly, they went underground--to the catacombs. The password to enter was to draw an IChThYS--in Greek (ΙΧΘΥΣ) .
This would be a fish because Peter and some of the other apostles were fishermen. It was called IChThYS the Greek Word for fish.
I — Iēsous (Jesus)
CH — Christos (Christ)
TH — Theou (of God)
Y — Huios (Son)
S — Sōtēr (Savior)
Jesus Christ, Son of God, Savior
