Why do so many Christians claim that other Christians are not really Christians? Is there a definition written in the sky somewhere?

Here we talk about the history of the problem, from its very beginning. Among other things we'll discuss why some biblical scholars today refuse to use the word "Christian" for the apostle Paul, the members of his churches, and for followers of Jesus in the first century altogether; and we'll ask whether there is something about "Christianity" in particular that leads to debates about what it actually means; and we'll try to work out if there's any satisfactory answer to the questions: "Is it possible to define the term to everyone's satisfaction?" and "Does it matter?"