For the term "Christian" to have any real meaning, there needs to be boundaries to it. It also isn't an organic term like "nerd" or "geek", there has, since the term was coined in the 1st century, a formal or semi-forma' organization that is involved with it. The Bible itself has instructions on organizing Churches and gives advise on leadership and the requirements for it. As such, there's ALWAYS been some sort of organization involved in defining who is and is not a Christian. Now, that said, the absolute head of the Church isn't the Pope, or the Bishops, it's God... In the end God decides who is truly Christian or not, but there's some clear guidelines in the Bible as well as rules for membership in Churches as well as discipline. The most simple and baseline requirement for being defined as a Christian likely is Romans 10:9 - "If you declare with your mouth, “Jesus is Lord,” and believe in your heart that God raised him from the dead, you will be saved." But declaring that carries a weight of expectations that involves a lot more... That can be looked at. And while yes, SALVATION is arguably personal, "being a Christian" involves more than simply that. Declaring that "Jesus is Lord", for instance requires you to understand who exactly Jesus is. And if you bear a false understanding who who Jesus is... Then who are you declaring Lord? It's not Jesus but something or someone else.