This is a very important question for us. I wasn't faced with this question near as much in the bay area as I have been here. I'm discovering that even in our neighborhood, this question has to be asked.
The following are excerpts from a post I read this morning that deals with this question.
This was the context in which I grew up. To put it charitably, most folks in my home town thought of themselves as Christians. Some evidently were by their profession and their lives. Some, no doubt, were not. But, most people didn't condemn themselves or think so poorly of themselves to self-identify as "practical atheist" or "unbeliever." Even the poolhall hustlers and the alcoholics claimed they knew God, "always said their prayers," and the "God knows their heart."
Many assumed the label "Christian" because they were certainly not practitioners of any other religion. They were not, for example, muslims. In fact, until I returned home during my sophomore year in college as a professing muslim, very few people had ever seen a muslim. No, everyone at least believed in some way, and that made most everyone "Christian" since that was either a family or community legacy bequeathed to all. Nominalism.
The more I think about the situation, the clearer it is to me that in trying to reflect genuine Christian distinctiveness in a nominal Christian culture definitions are critical. The root problem might be described as a failure to define "Christian" and Christianity in terms that bring into sharper contrast regenerate and unregenerate life, in terms that stress spiritual conversion, faith, grace, love and hope over and against moralism, patriotism, and spiritual relativism.