I suspect that any atheist who has brought up examples of Christian hypocrisy during interactions with Christians (and who among us hasn't?) has heard some version of the "not a real Christian" claim. The idea is quite simple: "real Christians" do not do bad things. Therefore, anyone who does bad things is de facto, not a "real Christian."
The Christian making such a claim is willing to ignore the entire body of evidence supporting the perpetrator's Christianity prior to the bad act because "real Christians" do not do whatever the perpetrator did. But there is another even more important way in which it is meaningful to discuss who is and is not a real Christian. There may even be a role for atheists can play in such a discussion.
If we view Christian belief on a continuum with liberal Christians on one end and fundamentalist Christians on the other, we typically see that each pole accuses the other of not being genuine Christians. Liberal Christians love to point out that fundamentalist beliefs emphasize a wrathful Old Testament god and miss the more compassionate character of Jesus. They also criticize the fundamentalists for refusing to allow their religion to evolve with the times.
On the other hand, fundamentalist Christians are fond of criticizing the "cafeteria Christianity" practiced by liberal Christians. They accuse liberal Christians of omitting whatever parts of their bible suit them and failing to honor the divinely inspired word of their god. These liberal Christians, they insist, do not even make a serious attempt to follow their bible.
Indeed, the tension between these two camps focuses on who has the right to regard oneself as a "real Christian." Each side views the other side as missing the point of Christianity and as not being true to the "holy" spirit, the central message, and the like.
I'd like to suggest that the part best played by atheists in this discussion is one of facilitator and critic. Simply put, we can encourage both sides to think. We can ask the liberal Christians how they justify ignoring the many parts of their bible with which they disagree, and we can ask the fundamentalists to consider the implications of a literal reading of the Christian bible in our modern world. We can ask the fundamentalist Christians why their god seems so angry and punitive when Jesus allegedly spoke of forgiveness, and we can ask the liberals why something "holy" seems to require so much interpretation. Perhaps provoking this sort of thought might help some Christians question the merits of their beliefs.
At this point, you may be asking yourself why atheists should even care about this debate within Christianity? In my opinion, those of us who live in a predominately Christian culture might care because it could be in our self-interest to do so. It seems that the nature of this debate could have implications for us, especially if various interpretations of Christianity continue to influence the laws under which we must live. The future of Christianity is relevant to us even as many of us hope to see a continued decline in its potency.
An earlier version of this post appeared on Atheist Revolution in 2009. It was edited in 2018 to remove typos and improve clarity. While I do continue to find many of the debates happening among Christians to be of interest, it is increasingly clear that the one over who deserves to be called a "real Christian" is not going to be resolved any time soon.