] The Greek spelling for the word fish is "ICHTHUS"
] (pronounced ikh-thoos) and was incorporated into the
] meaning of the fish symbol. ICHTHUS is an acronym coming
] from the first letters of the Greek phrase "Iesous,
] Christos, Theou, Uios, Soter" which spell out a secret
] message about our Lord. The individual letters I, X, O,
] Y, E, are the first letters of five Greek words each of
] which means "Jesus Christ, God, Son and Savior".
]
] I - Iota, the first letter in the Greek word Iesous, meaning Jesus
]
] X - Chi, the first letter in the Greek word Christos, meaning Christ
]
] O - Theta, the first letter in the Greek word Theou, meaning God
]
] Y - Upsilon, the first letter in the Greek word Uios, meaning Son
]
] E - Sigma, the first letter in the Greek word Soter, meaning Savior

I'd heard about early Christians drawing fish symbols as a secret identification sign, but I never knew the complete explanation...

Christian cryptography :)