Who is a Jew?

This is something that I wrote in a discussion board for a facebook group. It was on the topic of "if judaism is an ethnic group, can you be a christian Jew." Heres what i wrote:

I hope this doesnt offend any one or bore anyone:I dont know if youll find this relevant, but I argued with someone I know earlier. This is a very complex issue. Im still struggling with it myself, however, there is a concept in Judaism that you cannot convert out of Judaism, however that runs into trouble almost 600 years after 1492 when the spanish inquisition occured and even 60 years ago when the Holocaust occured because many Jews converted or hid their identity as jews for ever(possibly). Its alomost impossible to tell now(who is Jewish by blood), so what can one do, and the tough part is defining at what point you just "give up" and "decide" a persons(and/or their family) not a Jew anymore. On the topic of secular Jews, back in the Shtetls, If a Jew stopped being relgious, and I know thats a controversial term, they essentially left the community, and stopped being Jews. You either took what is now known as the orthodox path, or no path at all in the Jewish perspective. Ive come to the conclusion that outside of orthodox judaism, there is no such meaning of what a Jew is because every person has their own idea, and very few people come to agreement on this issue, so if you are a secular jew, there is almost no reason to worry about this concpet of a jewish ethnicity, becuase its whatever the secular jewish culture says it is. And secular jewish culture is very diverse and of course there is no agreement universally on this issue within that group of people who define themselves that way.

I thought this was very insightful and I i surprised myself with coming up with this idea...