The problem is: by whose definition?
God's. I'm not talking about which interpretation is correct, but if you agree that a Christian can be defined. Obviously, the Bible has something in mind when it uses that term. Just as it does for every term, and God has incorporated meaning to the words He speaks.
You may not consider me a Christian because I'm a Catholic, but others would. I do. I've read enough arguments between non-Catholic Christians to know this is a problem amongst yourselves too.
I believe the Bible defines what a Christian is, very clearly, and it has nothing to do with what church a person attends.
As a riff on the No True Scotsman fallacy.
Don't want to to take a detour here, but the "No True Scotsman fallacy" is fallacious.
No I'm not, there's no guarantee they will have better behavior because they're Christian. Just a hope that they would.
I would hope that everyone would have good behavior, and if they don't they should reap what they sow, but this has nothing to do with whether or not someone is a Christian, because, again, many unbelievers far excel in their behavior than those of Christians.
I'm not someone who will say former Christian, and maybe that's where the distinction lies.
Good, because this leads us back to defining what a Christian is, and I'm convinced that truth can be known, even if there are a plethora of ideas about what the truth is. Understanding what a Christian is does not take much study, but I find most people are too lazy to even look, or they simply don't understand what they are looking at, or as the Bible say, they don't have the ability to understand the things of God, because the Spirit of God does not dwell in them.
I don't have a particular standard of behavior that must be reached, I just look at someone's words and behavior and sometimes think they're a terrible ambassador. Not that they're not a Christian, just a lousy example of how a Christian should want to present themselves as a Christian.
The Bible is filled with evidence that a "believer" is identified by what they say they "believe", not by what they do, because no one can do it right, we all fall short. However, anyone can believe God, which is why Jesus said "this is the work of God, that you believe on Him whom He has sent". Jesus also said that we can know what is in someone's heart by what they say about Him out of their mouth.
I read the scripture, and I'll say right at the outset I can't argue it in a way you might be used to, so there's not much use in going that route. I have a vague understanding of the idea of being circumcised under the law vs. circumcised in the spirit but I'm not going to pretend I'm a Bible scholar. Nor do I think it's necessary for the purpose of this particular discussion.
Paul's point is that religious people make behavior or obedience to the Law an indicator of who is and who is not a Christian, but as Paul makes clear, that is a mistake, because if the unbeliever's behavior is better than yours, then he can claim to be a Christian, and you are not. If behavior is the evidence, then anyone who meets a certain standard of behavior could be called a Christian, and that's not what a Christian is. It's not adherence to Law, but a Christian is someone who has a new nature, from what they believe concerning Christ. Those who make behavior the evidence will cause unbelievers to blaspheme God, because they boast of their behavior and condemn others for theirs. Just read any of aCultureWarrior's posts.