Crosses are a sign of Jesus Christ Lord and Savior. It is the required symbol that Christians use to identify our faith in JC.
When the Christians were being persecuted in the old Roman times, they used the symbol of a fish because the Greek language for Jesus Christ Lord and Savior was ICHTUS, which also was a word for a fish. [I am writing this basically from memory and if anything I wrote is incorrect, please feel free to comment and correct me.]
Perhaps the anti-Christian crowd would like for Christians to go underground and use the fish symbol again? But we are not going gently into the night and will stand proud on our faith.