The early church during persecution used the fish as a way to communicate who they were without saying to much and getting reported to the authorities. There were times it could mean death the tell the wrong person you were a Christian.  I do not say using such a symbol was right or wrong, it is  just history.   The fish was used because in spelling fish in Greek  ἰχθύς , it is an acrostic for the names of Jesus and read as a statement it is true.

 ἰ      Ἰησοῦς     Jesus

χ      Χριστός  Christ

θ      θεός         God

ύ      υἱός          Son                 

ς       σωτήρ      savior

Jesus Christ, God’s  Son is our Savior.

This is only offered as some interesting and beautiful trivia you may use when you see the bumper stickers. I have asked many people “Do you know why the Fish is a Christian symbol?” almost always they don’t know. Then I have a door open to teach and witness.