I'm not going to answer that question today folks. It's been an open question for a number of centuries, or many of them, and this blogger can't resolve what lots of worthier sorts haven't managed.
Yet I will make an observation. There has been quite a hullabaloo recently, mostly among America's Jews, about how awful it is that various orthodox rabbis and secular politicians in Israel may or may not be about to legislate a law that would imply disapproval of some things some American Jews hold dear. The starting point of this hullabaloo is that American Jewish movements such as the Reform and Conservative denominations are totally legitimate expressions of Judaism, and any aspersions cast upon them are nasty, evil, backward-looking, parochial, primitive and diverse additional blemishes. Tablet Magazine even has a rather cute satirical piece by Shalom Auslander about The Day They Come to Take Your Judaism Away, though nothing in the logic of the discussion is remotely geared in that direction: the Orthodox complaint is mostly that the Jews are discarding their Judaism with fervent energy, not the other way around.
So just to put the matter in historical context. American Jewry in the past half century or so has radically redefined what it means to be a Jew, how you become one, what's expected of you as one and what's the relationship between the Jewish community and individual, and the surrounding society. Some of these changes may be for the better - assuming there's a way to define "better" - and others may prove to be fleeting innovations which leave no long-lasting mark. Yet for all the vagueness and ambiguities about how Jews decide about major changes to their way of life, the method chosen by American Jews is unprecedented. What has effectively happened is that various American Jewish groups adapted the practice of their religion to their immediate perceived needs, they made no serious effort to engage the rest of the nation, and now they shrilly demand that everyone else acquiesce.
Take the Clinton-Mezvinsky wedding. Not the personal aspects: everyone joins in wishing the new couple many years of the best. On a communal level, however, the entire affair flies in the face of much that many Jews would be willing to die for, and indeed, large numbers did die for over the centuries.
The more I reflect on the very serious discussion presently raging about Jewish identity, the more I'm puzzled by the supreme arrogance of some of America's Jews, who are unilaterally re-writing rules which were accepted for centuries, while reprimanding those of us who aren't greeting their project with enthusiasm. Odd.