This might just be a question of semantics. How does one define "Christian"? Mysterious has given us an accepted definition.
Some of the comments on this thread seem to be defining "Christian" by standards of behaviour: kindness, charity and so on. Which is either hugely derogatory to non-Christians, or meaningless - couldn't we just as easily say that a kind vegetarian JW is a Buddhist? Clearly not.
Christianity has to be defined by some belief or faith in Jesus Christ, and in my opinion, it is irrelevant if the believer is a complete bastard or a lovely kind person.
I tend to the opinion that if someone thinks that they are a Christian, then it isn't up to someone else to decide if they are a True Christian.