If the only way others can tell that your are a Christian is the symbol of the fish on your business card, do us all a favor and leave it off.
What does it mean?
In what other sense could we use it?
I believe what the person means is that if you do not act like a Christian (i.e., you don't represent the traditional Christian behavior in your everyday life) then don't make a big show about being a Christian by putting a symbol on your card.
In other words, if you don't live it, don't say it.
If you don´t mean what you´re doing better stop it now (leave it off).