It is literally impossible to define exactly what is a Christian
It cannot be done. That's because Christianity is so far from being a unified religion that there is too little common ground to use to establish a definition. About all one can say is that Christians worship a man/god who they believed walked the earth, said some stuff that other people wrote down later, and then was executed by the Romans about 2000 years ago and died. After three days, the story goes, he rose from the dead, talked to a few people, and then ascended into heaven.
Christians generally think that if you believe that story, all of your sins will be forgiven and you'll live forever in that same heaven after you die. The theory is that Jesus Christ sacrificed his own life in payment for everyone's sins. If you believe that, you're home free. You should also try to live a good life, of course, but even if you can't it'll be OK in the end.
There are lots more details. Different groups of Christians (there are thousands of such groups) believe all sorts of other things, and they argue a lot about those details. With hundreds and hundreds of millions of believers, such splits and different understandings of things were probably inescapable. But, all of those groups call themselves Christians, and who's to say they're not all saved. Certainly not me. I leave all that to them.
Christians feel all sorts of ways about all sorts of things. Just because you know someone who calls him or herself a Christian doesn't mean that you know much about that person. You have to wait to get to know them before you know how they feel about politics, charity, racial issues, money, sexual matters, and the like, just like with anyone else. Being a Christian doesn't mean anything about anything except what they generally believe about religious stuff. They believe different religious things, too, but that's way too complicated for me to talk about here.
It can be confusing, even for other Christians, really. But it's OK. They're all saved, so you needn't worry about that, assuming you were worried about that in the first place. You can expect them to try to get you saved, as well. That's up to you, of course. If you can believe the story, you can be saved, too, as far as I know. I can't believe it, but apparently some people can.
And that's all I can say about Christians. Really. Good luck in your encounters with them.
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There, it's a definition. I suspect it includes everybody who says they're Christians and excludes those who don't. You can't tell just by looking at a person, but then again you can't really tell what language they speak by looking at them. (Oddly, linguists have the same problem definition 'language', with lumpers and splitters duking it out as much as lumpers and splitters in taxonomics do ... Oy, Echinofossulocactus taxonomy!)
It's a general definition, and there are a lot of more fine-grained definitions, but they all miss part of {Christians} (I have no idea how the DU parser will interpret that { and } .)
Often the definition is chosen to exclude part of the set I have in mind, to redefine the set. Most often the other definitions are chosen without awareness or at least active consideration of the range of sectariansms.
Heck, on any given day in which I use the word "Christian" in 4 different conversations I'm likely to use 4 different definitions. Is my co-worker Christian? Sure and not in the least. Is my wife? Absolutely, but not really. Is the Pope? No way, but yeah. But the context had damned well better make clear what the distinction being drawn is.
But if you're going to go with all the distinctions as making no difference, then you're stuck with something like mine.
It's the same with "Democrat." Look at the struggles on DU to define the word. Each person gets to self-define. Doesn't mean DU has to let them stay. But the claim "you're not a true Democrat" always strikes me as specious and supercilious, boiling down to, "I get to define the term based on myself as prototypical, and you, sir, are no me!"
Might want to be a bit more selective?
