Recently on Facebook this article got linked to me. It refers to this other article titled "I'm Christian, unless you're gay." There was a reply to the Facebook post indicating confusion; the title of the article (I'm Christian, unless you're gay) was unclear. Initially I thought it was very strange too; is the author talking about changing religions based on who he is talking to? After a moment though it hit me that this was a use of an anachronistic use of 'Christian' that means being a good person. For a long time this was common usage in the Western world and you can easily see it when reading books from a century ago. Being a good Christian person is supposed to mean you are nice to people, treat them as you would want to be treated and forgive them for their crimes.
Of course this is a ridiculous little bit of conceit because I have never seen nor heard of any evidence that suggests that Christians treat other people better than any other religion you could name (same goes for philosophical stances that aren't religions like atheism). Neither treating other people well nor treating other people badly originated or was popularized by Christianity; it has no claim to moral authority based on history. Its claim of moral authority based on divine inspiration is of course much more contested though certainly equally false.
I think this sort of nomenclature is much like saying grace before meals. It is likely to remain in force within religions but is rapidly fading away from popular culture. The assumption of Christian membership is no longer reasonable and is quickly becoming less so as time passes and the ranks of atheists and 'ostensibly belonging to a religion but really agnostic/atheist' grow. Based on the author's writing it is clear that he comes from a highly religious family and area which would explain the usage of Christian in this sense. It is interesting to see that growing up in that sort of environment influences the words he uses so powerfully that his article titles can be difficult to understand for those without that background.