One of the signs for Christianity is the fish. The reason for this dates from some early societies of Christians who protected themselves and their congregations by keeping their meetings secret. In order to point the way to ever-changing meeting places, they developed a symbol which adherents would readily recognize, and which they could scratch on rocks, walls and the like, in advance of a meeting. The fish was chosen as eminently apropos.
The Greek word for fish, ikhthus (ichthys in borrowed words), spelled, in Greek characters, Iota-Chi-Theta-Upsilon-Sigma (ΙΧΘΥΣ looks like IXOYE in Roman alphabet characters) is regarded as an acronym for Ιησως Χριστος Θεου `Υιος Σωτερ (or, in Roman alphabet characters, Iesou (Jesus) Christos (anointed) Theou (of God) Uios (son) Soter (saviour)), which means Jesus Christ, the Son of God, the Saviour.
The constellation Pisces comprises a set of dim and scattered stars that trace the images of two widely separated fish joined by a knotted cord. One fish, swimming upward, faces east toward Aries, while the other fish swims westward toward Aquarius along the plane of the ecliptic. The directions of motion of the two fish form a cross, the symbol of the Christian religion -- the upright line of the cross representing spirit and the horizontal line signifying matter.
Babylonian mythology tells of two fishes that pushed ashore a giant egg, from which emerged the fertility corn-goddess Atagratis and her lover-son Ichthys, who dies and is reborn annually. The myth of Ichthys and the sign Pisces later became connected with Christianity. Directly across the zodiac from Pisces lies the sign of Virgo, symbolizing the virgin grain goddess of ancient Greece and also connected with the Virgin Mary of Christian mythology, whose birthday is liturgically celebrated on September 8, when the sun crosses the midpoint of the sign Virgo.
See also: Christian symbolism