JUDAISM OR JUDAISMS? Mark Goodacre writes, in part:

Is it just me or is there something rather annoying about the trend over the last twenty years or so to talk about early Christianity as "Christianities" and early Judaism as "Judaisms"? I must admit that I am hoping that this is going to prove to be just a fad and something that we will look back on in twenty years time as an odd terminological aberration that characterized a particular kind of scholarship at the turn of the millennium.

I understand, of course, the anxiety people feel about attempting to convey just how varied Judaism and Christianity were in this period, and it is natural to want to stretch language norms when one is examining emerging Judaism and Christian origins. But I can't help feeling that there is nothing helpful, interesting or insightful about this particular terminological oddity. If the variety of Christianity is not properly understood as "Christianity", then I don't see how calling it "a" Christianity in any way advances the discussion. It is either a variety of Christianity that might legitimately be described as a variety of Christianity, or it is not. Calling it "a" Christianity simply doesn't change anything. Likewise Judaism.

These are all good points. I think Neusner, who seems to have originated this usage, was trying to come to terms with a legitimate difficulty: the varieties of Judaism are so varied that sometimes it seems that some seem to have scarcely anything in common with some others, leading one to wonder what it is that makes them all Judaism. Nevertheless, I think he overcompensates. The fact that he uses the plural implies that there is some connection between the forms of Judaism, no matter how varied, and this requires explanation and description.

My favored solution is to apply Jonathan Z. Smith's "polythetic" approach to Judaism, which I summarized in a 2002 conference paper that you can read in full here:

In short, we come to an impasse: a "monothetic" definition of common Judaism in antiquity does not seem to work. That is, no definition of Judaism based on a sine qua non or core essence can be formulated. But Jonathan Z. Smith, in his article, "Fences and Neighbors: Some Contours of Early Judaism," has pointed to a way forward.[10] He proposes that classification of religions should follow a "polythetic" rather than a monothetic approach. Polythetic classification is an idea borrowed from biology. Rather than attempting to find an essence common to every member, it is based on a broad grouping of characteristics or properties. A member of the class being defined must have many of these characteristics, but no single characteristic is possessed by every member. Most members share characteristics with many other members, but some members have nothing in common with others. There will also be some borderline cases, which have a few of the characteristics but not enough to justify accepting these cases as members of the class. With a polythetic classification, the best we can do is to look for some general trends shared in antiquity by many or most Jews. Some important ones would include: worship of the God of Israel alone; acceptance of certain books as Jewish scriptures given as revelation by this God; the following of Jewish customs, laws, and rituals; participation in or support of the Temple cult in Jerusalem; self-identification with the Jewish nation; membership in and acceptance by a particular Jewish community; and acceptance of Palestine as the holy land. One can perhaps reasonably speak of a "common Judaism" and frequently shared elements, but there is no sine qua non. Some types of Judaism, notably the Enochic form, had very little overlap with common Judaism.

Mark continues in the same post quoted above:

And is it not the case that Christianity is far more diverse today than it was in the first two centuries? Many Christians within particular traditions do not recognise those in other competing traditions as Christians; there are Christian sects that are not called Christian by their founders; there are Christian sects that are not called Christian by their critics. We have every possible variety of belief and practice within contemporary global Christianity yet we don't feel the need to use the term Christianities. And likewise Judaism. Why, then, do we feel it necessary for the early centuries?

Again, all true. Superficially, Christianity seems to have a more obvious center than Judaism, because Jesus is centrally important to it. But on a closer look, people who consider themselves Christians often have disagreed widely (and violently) about who Jesus actually was and why he is important. A polythetic approach can be useful here as well.

In addition, as is becoming more and more widely recognized, the dividing line between Christianity and Judaism was very fuzzy to begin with and there was considerable overlap for a long time. And even after the "parting of the ways" Christians and Jews continued to talk to each other and influence each other from antiquity to the present. In all these cases there are continuums of variation rather than sharp demarcations.

For much more on all this, see the first chapter of my book The Provenance of the Pseudepigrapha: Jewish, Christian, or Other? (link above in the right-hand sidebar).