When people say "I'm a Christian but..." do you still think they're a Christian?
It depends what follows the but.
I've come across a few strange misconceptions about what Christians are like in the world, and more than a few people who are Christians who give the impression you have to be on your best behavior around them (I suspect at least some of those have their own strange ideas about having to be perfect, so act like they are perfect, resulting in others around them feeling like they need to be perfect as well).
If someone says "I'm a Christian but I still go to strip clubs and sleep with my secretary" that's a very different proposition to someone saying "I'm a Christian but that doesn't stop me going to the bar to watch the football game".
