Nationality and race.
Is that really true though? White supremacists would certainly disagree and say that being "white", however we want to define that precisely, is very much associated with a specific history, culture and institutions. That's after all why they keep using the term "western culture" in place of "white supremacy".
We easily dismiss this mode of making sense of the world as a transparent veil for racism. But we don't seem to apply that same scrutiny to nationalism.
It's also interesting to note that one might argue that both the concept of the nation and the concept of human races in it's modern form developed around the same time frame - the period when Europe transformed from a collection of fragmented kingdoms into nation states, which then started to colonise the globe.
I’m not sure how “western culture” can be construed as “white supremacy”. Europe, like Africa, Asia, North America, was never some melting-pot where everyone who had similar skin-tones carried a common culture into the future. The nations were often at war and sought to annihilate each other, even if by all outward appearances some of the soldiers looked alike.
Though it may have taken much time and bloodshed, “western culture” did much to overcome racism, slavery, discrimination, etc. as it did to spread it. The ability to criticize the past, learn from the mistakes and self-correct is one of the boons of being a part of it.