Why for all those centuries did no Christian realize this?
Apostle Paul (Ephesians 6:9) tells Christian slaveholders to be good masters. He does not say slavery is wrong.
You know full well, or at least I hope you do, that almost all, if not all, of those 'wars of religion' were in fact wars of [geo-]politics, with religion used to get the general population on board with what the ruler(s) wanted.
No different at all than 'freedom' and 'democracy' being used by the secular West in modern history, and 'egalitarianism' and Islamic solidarity by the Soviet Union and terrorists to get their respective populations in line with the government (or terrorist) stance. Freeing the slaves, protecting states' rights for the War Between the States/Civil War, etc. The real reasons for war are almost always far more down-to-earth and not particularly lofty. Ideology, be it religion or some other value, just helps make it more palatable to the masses.
It is horrible that religion was used as a tool in such a way - and fortunately our freedom of religion in the First Amendment helps protect us from that - but while the wars were waged in the name of religion, religion was not the root cause.
And if you consider yourselves to be fair, rational people, you have to acknowledge that religion has worked for good in secular affairs, too. Cases in point: the Roman Catholic Church almost singlehandedly is responsible for preserving writing and other knowledge in western Europe after the fall of the Western Roman Empire; monks recorded the histories and cultures of illiterate peoples - histories we might have no idea about today without that contribution; missionaries brought the pagan barbarians into the fold, encouraging them to settle down, not be (as) violent with raping and pillaging, and work constructively; and the institution served to help unify a fractured Europe, bestowing upon largely Germanic kings and nobility the authority to rule over their subjugated largely Latin/Celtic subjects by persuading the conquered to accept their new leaders - from a romantic point of view bad, from a peace/realist point of view, good; there's a reason that the old name for Europe was 'Christendom'. Christianity was the unifying factor, the sole one.
More to the point of this article specifically, it was religion, Christian religion, that provided the impetus to ending slavery globally. From what I have seen, Darwin was racist - though the same can be said for many, many freepers who delude themselves that they are not; Darwin may have been no worse than them. Even if his views were as rosy as those expressed here, he was by no means a major player in ending slavery. Religion, Christianity, specifically the Quaker variety, was.
And as opposed to religion being used as a tool in wars, the abolition of slavery was built from a grassroots movement of devout Christians themselves, not from some clergy-hierarchy or the state.
As for slavery being condoned, that is true. Yet the topic here is racism, not slavery. Roman era slaves were not considered subhuman or even separate species based on their ancestry. Once freed, they had all the rights, bar none, of born-free people.
While not endorsing slavery, there does seem an acceptance of the practice as fact. But also explicit is that all humans come from Adam, that all humans are of one family - which is linked to why Christ can save all of us - and that Adam was fully human.
So while you could argue that Christianity acknowledged slavery - the same way Christianity acknowledged Roman Imperial rule in the face of Christian persecution, mind you - you cannot make a strong claim that Christianity supported racism.
And, face it, by 'religion' most of the time you're (at least partly) referring to Christianity.
Life was extremely hard for most everyone except a privileged few. In certain situations slaves fared better than free individuals. In light of difficult circumstances for all but a few, slavery did not seem so terrible, relatively speaking.
As life became easier, the disparity between free and slave grew and then Christians saw slavery to be wrong.