It's called the 'No True Scotsman' fallacy. You can't legitimately argue that people do X because they're Christian, and when they don't do X, claim that because they don't do X, they're not Christian.
I mean, this is grade school logic.
Well done, laddie!
;-)
Scotsmen drink, therefore anyone who drinks is a Scotsman?
Or is it, if you are a Scotsman, you must drink?
Sorry, I acknowledge that there are Scotsmen who don't drink, but that does not mean that all drunks are Scotsmen.
You seem to have a problem understanding the difference between people who CLAIM to be Christian, and those who ARE Christian.