Patriotic-Jewelry.com/Ichthus-Christian-Fish

The fish symbol was a kind of grafitti code for the early christians.
Why fish? Well, if you write "jesus christ, Son of god, Savior," in greek, the first letter of each word spells fish, "icthys." And considering the significance of fish in jesus' parables and the various gospel stories, the symbol would be even more meaningful to the early christians.

I have been told that when two people meet in the early days the first would draw half a fish on the ground. If the other person finished the picture they knew that they were both christians, and therefore safe in each others company.