The Reform, and to a certain extent the majority of Conservative Jews, cannot be included in the historical definition of essential Judaism.
The problem in their regard is not one of practical and halachic differences, nor even in the acceptance or rejection of certain principles of faith.
Throughout the generations, all the Jewish streams had a unifying element: all parties based their opinions on the same corpus of source material, and accepted the same methods of interpretation.
It was thus always possible to argue, to debate.
The debate may have been about whether or not to include certain books in the Bible, or about the interpretation of Biblical verses or passages from the Zohar, but whatever it was, all the disputants based their arguments on one and the same material, even if their conclusions differed widely.
Today, too, there are also such differences of opinion on various issues of Halachah or faith
But the gap existing among today’s Jews – even among the believers – is such that dialogue on a common basis is practically impossible.
From this point of view, only Orthodox Judaism (and this term applies here not to any specific political or religious stream, but to a way of life and to a worldview) can “pass” this Jewishness test.
Strange as it may seem, in the existing situation, only a small minority of the Jewish people can be considered truly Jewish.
Hence the fundamental importance of the question, “What is a Jew.”
Is there, or can there be, a definition of Jewishness that will be applicable to the majority of Jewish people today?
The problem of defining what is a Jew in our generation is not only in finding a formula that will be valid, and yet as precise as possible for the greatest possible number of Jews.
An even more fundamental question is: in what way should such a definition be sought?
And even more profoundly, should we look for a definition that will be based on the Jewish present, or on the Jewish past?
In other words, should the essence of Judaism be defined in a historical or in a non-historical way?
The gap between the Jewish past and the Jewish present seems to make it impossible to find an historic solution.
Theoretically, it is possible to find characteristics, or ways of thinking, or abstract ideas that may define the essence of Judaism in our generation as it is, or at least as one can visualize its future development.
But is it indeed possible to find a definition of the essence of the Jew that is detached from Jewish history?
–Rabbi Adin Steinsaltz