The Ground Floor of the Skyscraper

An anonymous reader writes with a long series of questions, which I hope to answer perhaps at another time. Meanwhile, he raises one point I think worthy of consideration. In reference to the opposition of Christianity to slavery, which is a tradition stretching all the way back to Roman times, he says this:
As a historical note, it seems  important to recognize that although it was Christians who abolished racial slavery, it was also Christians who perpetuated it, and all of its dogmas. Suggesting that a creed is merited by the actions of its best members is as irrational as suggesting that it is demerited by the actions of its worst. It should be possible to evaluate Christianity as a religion without reference tothe deeds of its practitioners since they vary so widely in effect.
My reply: First, let us remind ourselves of the historical facts of the case; second, let us remember a creed is judged (among other things) on how it differs from other creeds, not on how it resembles them.
Christianity abolished slavery in Europe in the Middle Ages without regard to race, since even from the earliest days it was held to be unlawful for one Christian to own another: if a slave converted, it was required that he be manumitted. “Racial slavery” was practiced in the West in the Sixteenth Century, with Spanish plantations in the New World: the pope at that time issued a bull excommunicating any man who owned a slave. The bull was ignored by the worldly princes and wealthy landowners of the time. The actions and motives of William Wilberforce and the Abolitionist Movement of the British Empire do not need to be repeated by me in this place, since a popular movie has brought some of this history to the public attention. It is beyond dispute that these Abolitionists were Christians acting for a specifically Christian motive. The slave trade was ended, first in England, and then, through the blockades of the British Royal Navy, world wide.
The cultural confidence needed to enforce this standard against alien peoples is sadly lacking today, and so, in the Sudan and elsewhere, the slave trade is making its appearance. The confidence springs from Christianity. As the West detaches itself from Christian roots, it is finds the advances of Christendom slipping away and it wonders why.

I think it is also hard to dispute the fact that slavery was a worldwide institution, existing in all lands, all eras, all nations, and that the worldwide abolition of the slave trade was the single greatest moral triumph of human history.
Now then: certainly there were men who read (or misread) the Bible to support slavery. They needed excuses and so they invented them.
But a Hindu does not need an excuse; nor does a Mohammedan; nor does a follower of Buddha or Confucius or Lao Tzu; nor does any worshipper of any pagan pantheon. This is because these systems of belief and ways of life, noble as they are in other ways, condone and do not condemn slavery. The Jewish tradition on which Christianity is built does not condemn slavery. It was only when the Jewish idea that man was created in the image of God was combined with the Hellenic idea of isonomia, or equality before the law, that an idea specific to the Christian Roman Empire, where Jewish and Hellenic ideas had been combined, that this idea emerged.
So here is the error in reasoning: when judging the merit of a creed, one may indeed look at the actions of its best members, and the actions of its worst, and see whether the crimes of the worst are unique to the creed and caused by it, and see whether the virtues of the best are unique to the creed and caused by it. Because whatelse is a creed but a system of ideas promoting certain actions?
The mass-murders of the Nazis and the mass-expropriations of the Communists are unique evils of their regimes, because these actions were called for by their creeds. The Patriotism of the Nazis was not any more Patriotic than that of the allied nations, nor were they more brave or more inventive, nor did they show any other particular merit unique to National Socialism. A charity of the Communists toward the poor is significantly less than the analogous charity of the Capitalist countries: the workingman is much poorer under these regimes.
The Christians who owned slaves were no worse than the Mohammedans, Hindus, Pagans, and Chinese who own slaves. (Indeed, since the rights of the slave owner were restricted by laws, we can even say that to be a slave under a Christian was better than to be a helot under Sparta.) So much for the worse: they were like the rest of the world and followed the way of the world.
As for the worst, when Christians sin, they sin like all other mortals do, and for the same reasons, and using the same excuses. We are mortal men like all other mortal men. Even the outrages that seem to be specific to Christianity, the Albigensian Crusade or Spanish Inquisitions, turn out, upon examination, to be equal or better than the analogous Mohammedan Jihads or Pagan butcheries. An “Auto De Fe” committed in the name of Jesus is a crime against man and God, no question, but it is still better than the “Blood Eagle” of the Norse committed in the name of Odin, or the persecutions of Diocletian committed in the name of Rome. Accused heretics could recant and live, because Christianity, even in its darkest hour, was still a universalist religion. A victim of the Norse Vikings being hung on a tree and tortured to death as a sacrifice to Odin could not recant or join or become Norse, because this pagan religion is reserved only to those northern kings who have the blood of Odin in their veins.
As for the best, the Christians are the ones, the only ones, who promoted a moral code that condemns slavery. In this world of moral relativism, it is important to note that being anti-slavery is a culturally determined artifact. In other words, a Christian can say in all seriousness that slavery is objectively wrong because it is against God’s will; but anyone else who says slavery is wrong can only say slavery is subjectively wrong, and this is an opinion he hold merely because he is a member of Christendom and has absorbed the values and standards of Christendom. An economist might point out that slavery is innately inefficient; but even an economist would not call it “wrong” in situations where its inefficiency was counterbalanced by some corresponding economic advantage; economists do not make judgments of right and wrong. The idea that human life has an innate dignity is a Judeo-Christian idea.
In short, we see Christians acting badly for the same reasons all men act badly; but we also see Christians acting virtuously with virtues no other creed proposes.
So my anonymous reader tells me it is”irrational” to judge a creed by seeing it at its best and worst. The abolition of slavery looms like a skyscraper on the skyline of history, as the only truly and unambiguous advance in the morals and manners of mankind since the dawn of time.
In effect, my reader is telling me the skyscraper cannot be taller than the two-story house, because the ground floor of both the house and the skyscraper are on the same level.