When threatened by Romans in the first centuries after Christ, Christians used the fish 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.
Greeks, Romans, and many other pagans used the fish symbol before Christians. Hence the fish, unlike, say, the cross, attracted little suspicion, making it a perfect secret symbol for persecuted believers. So the early Christians made practical use of this symbol for practical convenience. It is somewhat similar to the use in our days of bumper-sticker and business-card practice to be recognised by strangers, although we are not yet under persecution.
As early as the first century, Christians made an acrostic from this word: Iesous Christos Theou Yios Soter, (ICTYS) i.e. Jesus Christ, Son of God, Savior, using the Greek word for fish "ichthys." The Greek word Ichthus (Iota Chi Theta Upsilon Sigma), pronounced ich-thoos, upper case: and lower case: , is the word used throughout the New Testament for the English word fish.
In pagan beliefs, Ichthys was the offspring of the ancient Sea goddess Atargatis, and was known in various mythic systems as Tirgata, Aphrodite, Pelagia, or Delphine. The word also meant "womb" and "dolphin" in some tongues, and representations of this appeared in the depiction of mermaids. The fish is also a central element in other stories, including the Goddess of Ephesus, as well as the tale of the fish of the Nile that swallowed part of Osiris' body (the penis), and was also considered a symbol of the sexuality of Isis for she had sexual intercourse with Osiris after his death which resulted in the conception and birth of his posthumous son, Harpocrates, Horus-the-child. So, in pagan beliefs, the fish is a symbol of birth and fertility.