For many pop-culture Christians, the "fish" decal on the back car bumper, or attached to a key chain or door is a symbol of their religion, and a feel-good statement about Jesus Christ. Early Christians used the fish as a recognition sign of their religion. It is also identified as the "Ichthus," an acronym from the Greek, "Iesous Christos Theou Uios Soter," or "Jesus Christ the Son of God, Saviour." Oxford English Dictionary (C.E.) defines "Ichthyic" as "of, pertaining to, or characteristic of fishes; the fish world in all its orders."

But contemporary Jesus worshippers might be surprised, even outraged, to learn that one of their preeminent religious symbols antedated the Christian religion, and has its roots in pagan fertility awareness and sexuality. Barbara G. Walker writes in "The Woman's Dictionary of Symbols and Sacred Objects," that the acronym pertaining to Jesus Christ was a "rationale invented after the fact... Christians simply copied this pagan symbol along with many others." Ichthys was the offspring son 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 also a central element in other stories, including the Goddess of Ephesus (who has a fish amulet covering her genital region), as well as the tale of the fish that swallowed the penis of Osiris, and was also considered a symbol of the vulva of Isis.

Along with being a generative and reproductive spirit in mythology, the fish also has been identified in certain cultures with reincarnation and the life force. Sir James George Frazer noted in his work, "Adonis, Attis, Osiris: Studies in the History of Oriental Religion" (Part Four of his larger work, "The Golden Bough") that among one group in India, the fish was believed to house a deceased soul, and that as part of a fertility ritual specific fish is eaten in the belief that it will be reincarnated in a newborn child.

Well before Christianity, the fish symbol was known as "the Great Mother," a pointed oval sign, the "vesica piscis" or Vessel of the Fish. "Fish" and "womb" were synonymous terms in ancient Greek,"delphos." Its link to fertility, birth, feminine sexuality and the natural force of women was acknowledged also by the Celts, as well as pagan cultures throughout northern Europe. Eleanor Gaddon traces a "Cult of the Fish Mother" as far back as the hunting and fishing people of the Danube River Basin in the sixth millennium B.C.E. Over fifty shrines have been found throughout the region which depict a fishlike deity, a female creature who "incorporates aspects of an egg, a fish and a woman which could have been a primeval creator or a mythical ancestress..." The "Great Goddess" was portrayed elsewhere with pendulous breasts, accentuated buttocks and a conspicuous vaginal orifice, the upright "vesica piscis" which Christians later adopted and rotated 90-degrees to serve as their symbol.
http://www.atheists.org/christianity/fish.html