“It is well-known, that the earliest Christian emblems – before it was ever attempted to represent the bodily appearance of Jesus – were the Lamb, the Good Shepherd, and the Fish. The origin of the latter emblem, which has so puzzled the archeologist, thus becomes comprehensible. The whole secret lies in the easily ascertained fact that, while in the Kabala, the King Messiah is called “Interpreter”, or Revealer of the mystery, and shown to be the fifth emanation in the Talmud – for reasons we will now explain – the Messiah is very often designated as “DAG”, or the Fish. This is an inheritance from the Chaldees, and relates – as the very name indicates – to the Babylonian Dagon, the man-fish, who was the instructor and interpreter of the people, to whom he appeared.
Abarbanel explains the name, by stating that the sign of his, (Messiah’s), coming “is the conjunction of Saturn and Jupiter in the sign Pisces.” Therefore, as the Christians were intent upon identifying their Christos with the Messiah of the Old Testament, they adopted it so readily as to forget that its true origin might be traced still farther back than the Babylonian Dagon. How eagerly and closely the ideal of Jesus was united, by the early Christians, with every imaginable kabalistic and Pagan tenet, may be inferred from the language of Clemens of Alexandria, addressed to his brother co-religionists.
When they were debating upon the choice of the most appropriate symbol to remind them of Jesus, Clemens advised them in the following words: “Let the engraving upon the gem of your ring be either a dove, or a ship running before the wind, (the Argha), or a fish.” Was the good father, when writing this sentence, laboring under the recollection of Joshua, son of Nun, (called Jesus in the Greek and Slavonian versions); or had he forgotten the real interpretation of these Pagan symbols?
Joshua, son of Nun, or Nave, (Navis), could have with perfect propriety adopted the image of a ship, or even a fish, for Joshua means Jesus, son of the fish-god; but it was really too hazardous to connect the emblems of Venus, Astarte, and all the Hindu goddesses – the argha, dove, and fish – with the “immaculate” birth of their god! This looks very much as if in the early days of Christianity, but little difference was made between Christ, Bacchus, Apollo, and the Hindu Christna, the incarnation of Vishnu, with whose first avatar this symbol of the fish originated.
In the Hari-purana, in the Bagaved-gitta, as well as in several other books, the god Vishnu is shown as having assumed the form of a fish with a human head, in order to reclaim the Vedas lost during the deluge. Having enabled Visvamitra to escape with all his tribe in the ark, Vishnu, pitying weak and ignorant humanity, remained with them for some time. It was this god who taught them to build houses, cultivate the land, and to thank the unknown Deity whom he represented, by building temples and instituting a regular worship; and, as he remained half-fish, half-man, all the time, at every sunset he used to return to the ocean, wherein he passed the night.
“It is he”, says the sacred book, “who taught men, after the diluvium, all that was necessary for their happiness. One day he plunged into the water and returned no more, for the earth had covered itself again with vegetation, fruit, and cattle. But he had taught the Brahmas the secret of all things.” (Hari-purana). So far, we see in this narrative the double of the story given by the Babylonian Berosus about Oannes, the fish-man, who is no other than Vishnu – unless, indeed, we have to believe that it was Chaldea which civilized India!”
H. P. Blavatsky
The Mark Of God: Understanding Your "I AM Presence" 