There is no such thing as an "ethnic" Jew any more than there is an ethnic Baptist or an ethnic Lutheran. Judaism is a religion, not an ethnicity. Although Judaism began with the Semitic tribes (they are an ethnicity), it spread to other ethnicities as well, such as the Khazars of Kazakhstan through conversion in the ninth century.
And a Semitic Jew is not always a Jew. They may convert to Christianity or Islam as well and believe as deeply and totally in their new religion as a devout Jew does in Judaism.
Let me ask you this: If one is born a Semitic Jew, at what point is he/she expected to embrace the religion before he/she is classified as not really a Jew? And why is it that so many Jews still consider them Jews?
Oh...and, how far off pure, traditional Judaism can the fellow/gal get before they are no longer considered a Jew?