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.