Let me explain. There is something inescapable about the culture associated with this particular religion. While I can only speak from my experience as a Jewish woman, I don't doubt that other religions can claim their own cultural connections as well. Jews are often referred to as being a race of people in a way that doesn't necessarily apply to people of other faiths. While I definitely cringe at the word "race", my own experience does point to a strong set of mores, traditions and even personality traits shared by Jews that go way beyond religion.
Please forgive my gross overgeneralizations; while I don't presume to speak for all Jews, I'm just going to write as if I do. When you're born into a Jewish family, you begin to participate in a kind of shared experience that automatically gives you something in common with every other Jew on the planet (so to speak). The big dinners complete with bubbie's chicken soup. The enthusiastic "Mazel tov!" upon hearing any piece of good news. The nagging questions about why you're single and when you're getting married, plus countless unappealing offers to be set up with "my friend's grandson - a doctor!" The automatic sensitivity over historical marginalization, the Holocaust, and the state of Israel. The (infuriating) assumption that you will marry a Jew.
There is no doubt that Jews stick together; we are exclusive to a fault. One thing that I do appreciate about this members-only approach is that it is one of the few religions that does not attempt to proselytize. Unlike Christianity and Islam, Jews do not try to convert the masses just for the sake of making our numbers more impressive. Instead, Jews prefer to band together and stay that way. I'm pretty confident that this strange bond and resistance to outsiders reaffirms my theory that Judaism is more of a cultural identity than a religious one. Others aren't welcome because even if they go to synagogue on the right days and say all the right prayers, they still won't possess that ineffable quality that makes someone a Jew.
Unless you try to marry a non-Jew. Then he/she had better convert. ASAP.
In any case, whether you're Jewish or any other religion under the sun, it is very difficult to remove yourself completely from the influences of your upbringing. When I decided to tell my family about being an atheist, they were very afraid that by not believing in god I was also rejecting the values and traditions instilled in me. Is it possible to separate these things from a purely religious context? Can I be culturally Jewish but religiously atheist without undermining my core beliefs? Do I even want to hold on to the cultural ties that are born out of principles to which I am vehemently opposed? Most importantly, I am trying to figure out how to live my atheist life without alienating myself from friends and family... even if they do believe in god.