I know there is a book entitled that, but this isn't about the book. From my limited experience with talking to Jews about their heritage, I am told there are two "types" of Jews. There are those Jews who refer to themselves as Jewish because they practice Judaism and there are those who refer to themselves as Jewish because they are descendants of those people.
Speaking to one non-practicing "Jew," she said she referred to herself as Jewish (even though her family was from Germany) because her family was shuffled around from country to country and never felt any identity with a nationality other than a unity shared by Jewish people.
Then there are others who simply say they are Jewish because of their religion, regardless of where they come from.
So, on the character issue, some may be Jewish in the sense of that identity, and others may be Jewish because of their religion. I suspect Jerry is somewhere in between. George I think is agnostic. Kramer...God, who knows? Elaine, well, I think she's agnostic as well. The cross, the "I'm not Jewish", the "I don't know what a kosher meal is", the "I'm not of your faith"... I think it all falls into place.
But, I'm not Jewish and my simple examples point to that fact. On the other hand, this is how I think based on what certain Jews have told me...