[drc500free]

A less charged version of this is Daniel Boyarin's take in "Border Lines," that early Christianity and early Rabbinic Judaism were largely defined in opposition to each other. I.e. that being "not that other thing" both established a bright line division where one didn't previously exist, and shaped the things on each side of that line. (And in his opinion, created the very concept of "a religion" as a distinct package of culture and ideas where previously it was more integrated across social activities and behaviors).

As Christianity became dominant, it began to schism internally with much the same pattern, fractally defining parts of itself as things like "not Arian" or "not Catholic." Judaism, embedded within Christian society, still primary organizes around being "not Gentile" which ranges from simply Christian to full blown anti-semitism and pogroms.

[hutzlibu]

"that early Christianity and early Rabbinic Judaism were largely defined in opposition to each other"

I never heard of that theory, but I don't think it makes a lot of sense. I would think early christians mainly defined themself by believing in jesus christ, meaning they believed Jesus from Nazareth was the Messias and the other jews did not believe jesus was the messiahs.

And then you had christians who believed jesus resurrected from the death and those who did not. Then you had those who believed it was only a jew thing and then you had Paulus, who made it a universal religion, ... so all in all, plenty of different things people believed in. So surely some groups of people define themself by what they are not, but I don't think this was valid of early christians.

[WillPostForFood]

"they believed Jesus from Nazareth was the Messias and the other jews did not believe jesus was the messiahs"

Isn't that definition in opposition?

[hutzlibu]

Not in my understanding: it is

group A believes in X

group B believes in Y

What the parent poster seemed to imply was

group A believes in not Y.

Or a more concrete example of today, many people today define themself by being anti green, anti progressive, anti woke, antifashist etc. but often struggle to define what they are standing for.

[drc500free]

The theory would be that given A believes X and B believes Y, A is more likely to incorporate ~X and B is more likely to incorporate ~Y.

And because beliefs tend to be on a bit of a continuum, what starts with A believes Z <= 4 and B believes Z >=5 evolves to A believes Z <=1 and B believes Z >=9, and both believe anyone who believes 3<=Z<=7 is not part of their group.