Then again, he had to support his thesis.
Racism is an instance--a rather peculiar one--of the kind of tribalism that you see in pretty much every society. When my half-brother's grandmother said, "Ah, those Calabresi, they're all so stupid they can't even speak right" she wasn't being racist she was being tribal. She was Sicilian.
Line up skin color with tribal boundaries and you're part of the way to having racism. But you need to then pitch in some sort of rationalization. Simple "the worst in my group is better than the best in their group" won't do it. For that you need two things. The first is a strong push to consider all men as equals. The second is a strong reason not to consider blacks as equals. Then you have a neat little syllogism.
"All men are equal.
"Blacks are not equal.
"Blacks are not men."
The belief that all men are created equal, the belief that we're all equal in God's sight, is a valid example of the first kind of push. Making matters worse was that as time went on, the idea that we should be kind to others mitigates against even the usual, older, non-race-based view of slavery. It's hard to have even OT-slavery or Greek-style slavery given Deist or theist views of the NT common in the 19th century. (No problem in the 15th or 16th, mind you.)
The strong need to maintain blacks as property for the economic system was a strong reason not to consider blacks as equals. Avoiding the condemnation implicit in being your brother's keeper and treating him badly is another reason not to view him as your brother. Moreover, as soon as there's an argument saying you're wrong about virtually anything there's another strong reason to argue that blacks aren't equals--you are in an argument and need to see yourself as right and your opponent as wrong.
At that point you don't actually have to buy the reasoning any more. It's just a fact that you've accepted. Somebody else has justified it, it's okay. That's how most people treat most doctrines, whether religious, political, or economic. The good ol' appeal to authority in lieu of understanding the argument and evaluating it.
(At this point, most racism, IMHO, isn't due to this kind of syllogism. It's due to small number statistics and how we evaluate incomplete data in the light of infrequent events. Combined with in-group/out-group biases.)
Muslims had no problem dealing in slaves. More blacks were taken as slaves by Muslims than in the trans-Atlantic slave trade. Yet their racism is different from American racism, even if there is still a race/class-based racism that's superimposed on out-group religious bigotry in many cases. Their boundaries weren't primarily ethnic or based on skin tone (even thought that's present), but religious. So that even under the Ottomans your "ethnicity" was things like "Christian" or "Druse".
Marx had an improverished set of categories and values. Many current thinkers share his impoverishment. "Blessed are the poor" is not an appropriate statement for this kind of poverty.
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