just a matter of what you think the most important way to see the world is. some think it's race, some income, some religion, some nationality, others politics, etc. Marx hated nationalism and religion (the opiate of the people) because they distracted people from what he thought was the most important way to divide society - along class lines. who's to say which division captures society the best? personally i don't put too much stock into religious differences, but i do think economics (income, class, etc) is an important division to pay attention too.
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"if you tremble with indignation at every injustice then you are a comrade of mine."
- Che Guevara