LOL -- this I've got to hear...
By Christ's time, the fish had been in use for over a thousand years as a symbol for the Great Mother Goddess in many pagan religions. The whole represents the goddess' vulva (put it on it's end) and therefore fertility, good harvests, etc., while the two crescents used to draw it represent the moon, very important to pagans.
The cross, or versions of it, also had earlier origins with the Assyrians and Egyptians.
Come to think of it, this symbol-hijacking has a more modern incarnation. Hitler used the swastika, long a religious symbol usually connotating peace and love and other good things, for his Nazis. And the pentagram used to mean something good to Christians and Jews.