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.
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.
Isn't that definition in opposition?
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.
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.
Children under the age of 15 in Palestine make up 50% of the population. In Gaza, it's 45%. One thing I've noticed is news media equating Hamas with the majority of Palestinians, they weren't even born when they came into power.
Apartheid Israel and Palestine are not the same. Israel is the aggressor. They've ethnic cleansed so many Palestinians that there are 5.9 million refugees in Jordan and Lebanon. That's half of Israel's population.
* Illegal Settlers living in Palestine can vote in Israel but not Palestinians. Apartheid South Africa did the same, they put the people in their own "country" and so couldn't vote. Israel doesn't want 1 state as that would mean millions more voting. And it doesn't want 2 states as that would mean giving up valuable land.
* Israel has ethnically cleansed and fragmented areas into [isolated cantons divided by Israeli settlements](https://en.wikipedia.org/wiki/Palestinian_enclaves), and implemented [lebensraum](https://en.wikipedia.org/wiki/Lebensraum) tactics. This was called [Bantustans](https://en.wikipedia.org/wiki/Bantustan) in South Africa.
* [It denies Palestinian fishing past 3 nautical miles in some areas, and not past 15 nautical miles](https://www.ochaopt.org/sites/default/files/styles/inline100...)
* [The Apartheid Separation wall](https://i.pinimg.com/originals/d0/73/81/d07381fffef632350bcb...)?
* The settlers kill and burn homes to get rid of people: [stealing homes](https://www.youtube.com/watch?v=KNqozQ8uaV8) and destroying olive tree groves. They pour concrete into Palestinian farmers' water wells.
* [They control the water completely](https://en.wikipedia.org/wiki/Water_supply_and_sanitation_in...).
* [They get 12 hours of electricity a day](https://www.ochaopt.org/page/gaza-strip-electricity-supply).
* Palestinians didn't have 3G until 2018, their access is also restricted.
* They [blockade](https://en.wikipedia.org/wiki/Blockade_of_the_Gaza_Strip) Gaza, making it a concentration camp of 2.2 million people where 44% are children.
* [Not All Israeli Citizens Are Equal](https://www.nytimes.com/2012/05/24/opinion/not-all-israeli-c...)
* There were `5,248,185` Palestinian refugees in 2020; that's equal to half the population of Israel.
* [Israel dictates who you can marry](https://preview.redd.it/1k7jnev0pq971.jpg?width=1024&auto=we...)
* [Segregated road system](https://s3.amazonaws.com/VP2/visuals/en/4db97391fe7cb2462249...)
* [Israel Forcibly Injected African Immigrants with Birth Control](https://www.forbes.com/sites/eliseknutsen/2013/01/28/israel-...)
* 50% of Palestinians are children.
As someone from South Africa, I've seen ethnic cleansing and Israel is doing worse. Israel is absolutely an Apartheid state. Various human rights organisations have already stated this.
But you are arguing disingenuously, you know it, and I'd rather not continue this farce.