In my short stint attending church, I asked my pastor (still a good friend) what this meant, and what the significance of the fish was. He said that in Roman times (I assume around 50 BC to 50 AD), a Christian, when meeting, would draw one half of the fish () (rotated 90 degrees, of course), and the other Christian would complete the fish >, confirming eachother were Christians.

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.( )
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(Best I can do without gaining much ASCII Art skillz and an aneurism.)

Now, as to why people feel the need to announce their Christianity to the world via this car emblem, well...that's another node. Not very humble of them though, is it?

Kindtaro: I agree with you for the most part, on the Darwin fish, but taken at the same level, the Darwin fish is the same as the ichthus operated way back when, identifying one's beliefs. People are too uneducated to have any idea what that fish means beyond "oh, they're Christian" (in general, at least). If it were, say, a swastika with "darwin" circumscribed somehow, that would be a different story...