As others have indicated, premodern slavery really had little to do with racism. Also, European "racism" in the modern sense really did not exist prior to the age of exploration. Before then, most East Asians, South Asians, and Africans that Europeans contacted came from societies that were as technologically sophisticated and highly organized as their own. Like all people, Europeans may well have created reasons to stereotypically hate or distrust these peoples on cultural or religious grounds, but the notion of biological inferiority was a later developmeent. Had substantial populations of black africans or middle easterners been brought into Europe by the Romans and later become Christians, I believe their presence would have been generally accepted, and that they would not necessarily be enslaved or placed into second-class citizenship because of their race alone.