hokie is correct. The early Christians, persecuted by the Romans,
used the "X" and the fish sign as a sort of code.
cheers
PS Guinness is good for you and Merry Christmas. BTW, the clerk
at CVS Drug wished me "Happy Holidays", so I asked which holiday
do you mean, Valentines? She said, "Christmas" so I replied,
"Happy Christmas to you too" and she gave me a sheltered thumbs
up as if to say that she was not allowed to say that.
I guess we will have to go by code soon.