A Jew is Still A Jew

Roman Catholicism is the largest of the three major branches of Christianity. Thus, all Roman Catholics are Christian, but not all Christians are Roman Catholic. Two lit candles in Roman Catholic Church of The Assumption, Peekskill, New York.

Christianity and Islam, as world communities, are alike. Both were conceived naturally but as intentional communities. With 2 billion Christians and 1.5 million Muslims where millions are born into one community or the other without little given choice, none of them began as birth-based, and neither has entirely forgotten its origins. 
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Their unique strength relies on the fact that anyone can join them, anyone can convert to one or the other, for they are Abraham’s spiritual progeny. Their unique weakness is that anyone can quit them; anyone can apostatize. 
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Birth-based communities cannot grow easily, but neither can they shrink so easily. Thus, a Jew who is not a Judaist—not a practitioner of Judaism—is still a Jew.  

There is the difference. 

Source:  God in the Quran by Jack Miles