This is really for one kind of reader: Atheists. Often a skeptic will cite some particular Christian, or group of Christians and hold them up as a representative sample of Christians and Christianity. There also exist sects of people who claim for themselves the label of “Christian” yet are largely dismissed by the broader body of believers due to affirmations or denials of certain doctrines traditionally held by Christians.
For instance, the Westboro Baptist Church, those who were responsible for the Inquisition, or even the Jehovah’s Witness and LDS groups. I can also think of times where a skeptic will hold up some group of politically liberal professing Christians as being the standard that more professing Christians should be strive to be like. This usually includes groups accepting of homosexuality and same-sex marriage, and some form of pluralism — that you don’t necessarily need to be a Christian to be saved.
Of course, the defender of Christianity will object to their being held up as “Christian”. They merely identify themselves as Christian, but their affirmations and actions disqualify them as being “true” Christians, or so some say.
To the Atheist: can it be the case that someone or some group of people self-identify as Christian yet are not actually Christian by theological and doctrinal definition? Who or what determines who is and isn’t a true Christian? Because Christianity is a belief system, there are certain required beliefs and required denials. Therefore, doesn’t it stand to reason that not everyone is a true Christian simply by virtue of claiming it?