slavery as we understand it is actually primarily about religion, and this was developed by the muslims, but in europe and not in africa. when the romans enslaved people, it was largely about class. the muslims changed this by converting the basis of slavery to a question of whether one was muslim or not. so, when they enslaved the ukrainians, it was not because they were white or even because they were christian but explicitly because they weren't muslim. there is some remnant of this today in isis' declaration to non-muslims to convert or be enslaved (or die...) - that is more or less how the process of islamic expansion occurred in a very wide area of the world. when they showed up on the shores of europe and raided european cities for slaves (and they did not just raid rome and marseilles, but also raided london and paris), they did it under the justification that they were non-muslims. when they built their janissaries, it was to spread islam by force.
european slavery happened as a reaction to islamic slavery, and stems not from an aristocratic declaration of ownership or a corporate drive for profit but from a papal bull declared in 1452 - 40 years before columbus, and directed primarily at the western coast of africa. the christians were upset that the muslims could enslave them based on their religion, but they couldn't enslave the muslims based on their religion. they saw this as unfair, and wanted reciprocity. so, the following papal edict was declared to address the point:
We grant you [Kings of Spain and Portugal] by these present documents, with our Apostolic Authority, full and free permission to invade, search out, capture, and subjugate the Saracens and pagans and any other unbelievers and enemies of Christ wherever they may be, as well as their kingdoms, duchies, counties, principalities, and other property [...] and to reduce their persons into perpetual servitude.
with that declaration, christian explorers were now allowed to take slaves of arab and african, and eventually native american, populations - not because of what they looked like, but because of their beliefs. with that edict, the christian declared the rest of the world it's slave.
you may argue that this is a kind of convenient fiction, for surely the christian never found a mirror-image in africa! but, you would be wrong. in fact, this policy was tested on the ethiopians, who were christianized in the pre-islamic era, presumably by romanized copts moving south along the nile. they apparently had a memory of rome, although the portugese had no idea they were there and were flatly shocked to find them. but, there was no raiding of ethiopian villages for slaves. europeans never colonized them - not until the italians invaded them in the 1930s. that's a distance of almost five hundred years, in which ethiopia maintained independence while the rest of africa was invaded. the portugese actually signed a mutual defense treaty with them against their common enemy, the saracens.
so, it's important to understand the nature of slavery in the pre-modern era as being explicitly religious and ultimately of islamic origin, imported by the pope so that christians could compete with muslim slave raids. and, this remained the basis of slavery in the christian world for centuries.
it was only when the puritans in america started questioning the logic of things and pulling out contradictions in the reasoning that race was presented as an argument, and even then it was to uphold an existing economy that slave owners did not want to reform. everything else aside, including questions of papal authority, the reality is that they were christianizing the africans, so how could it be claimed that they should be enslaved? isn't the premise in opposition to pretty much every christian principle that there is? in an attempt to quell a potentially dangerous dissent, the slave owners turned their own book on the earliest abolitionists: it is stated in genesis that the descendants of ham should be punished. that's why they had burnt skin and flat noses. they could cite aristotle for back-up.
so, it was only after the system existed and the brutality of it was questioned that race was brought in to justify it. preachers were told to go out and teach this, and it disseminated through the church from that point. as the black african must not just be enslaved but also punished, the brutality of the system increased - not just for economic reasons but for perceived moral ones.
in time, this lost effectiveness, and most people know the history form there. when religion failed, they tried to use science. it's always been a means to an end. but, the effects of this cynical reading of christian scripture to uphold an economic system have never been fully erased or resolved.