Elisheva's comment reminded me of something that used to be true, but I don't know if it still is. For good historical reasons, American Jews used to regard the words Christian and Gentile as interchangeable in everyday conversation. This is because the overwhelming majority of Jews were of Ashkenazic origin, from Europe. All European countries were at least nominally Christian of some sort, and the religious concerns of Christians tended to dominate the culture there.
Similarly, when Americans referred to Jews they meant Ashkenazi Jews from Europe, though most gentiles don't know that distinction. But even among those who knew that Sephardic (corrected: Oriental) Jews existed, or were aware of smaller communities in such diverse lands as Ethiopia and India, Ashkenazi Jews from Europe, especially Central or Eastern Europe, were what they were thinking of when they thought of Jews. They were pretty much all that were here.
If you pressed them on the issue, both groups were aware that the generalization was sloppy and inaccurate , but everyday conversation is usually not taken up with such overprecision. I even recall two Jewish friends telling me that their own Jewish friends would sometimes look at them oddly when the consciously avoided saying "Christian" in conversation and used "gentile" instead. (One was a closet Buddhist, the other may have just learned to make the distinction for my sake.)
I thought this would slowly vanish, as everyone became more secular and everyone became more aware of the Middle-East. Yet I have no evidence that this has actually happened. Perhaps among younger Americans, both gentile and Jew, who are more used to friends having no religion at all, or a religion that is neither such as Islam or Hindu, this is changing. Also, both gentile and Jewish Americans have traveled to places other than Europe now, including Israel, Turkey, Egypt and other spots where there are Oriental Jews. Both factors would work to undermine the mental picture of Jewish=Ashkenazi and gentile=Christian. People my age might still keep the mental pictures and sloppy generalizations of our youth.
I have a couple of people I can ask about this, and I would appreciate if you all would as well.
Update: Don't neo-nazi groups use "Christian" in the same way, to mean "not Jewish?" Do they still do that? Fundamentalists are more likely to use it to mean "Not secular; you Jews are like, sort of okay."