Someone complained to me recently about an erstwhile friend who was acting like a self-centered jackass. In the course of the discussion, the friend was described as "not a very good Christian." It seemed to me that she wouldn’t have been a very good Buddhist or Taoist, either!
Do Christians have the market on "goodness?" What one group needs that kind of pressure, to be the world’s goodness guardians? And, what of everyone else? Not withstanding your religious beliefs, most of the world is not Christian, and so are they all bad? To characterize someone as a less-than-perfect Christian is less descriptive than to say "not a very good/nice person," which is what you generally mean to say anyway. I can only assume that, as a species, we’re more comfortable judging the quality of another’s Christianity, and not the quality of their humanity.
Of course, I could be wrong. 🙂