I, also, like Moonbo, don't enjoy entering debates like this for the same reasons he noted. I especially hate to pop in just for this after I've taken a long hiatus from TDM. However, I'm quite disappointed in some of the dead horsiness around this subject.
But it did condone more righteous, generous behavior towards slaves.
Which also means that it condones slavery. Contradicting your claim that it "made it immoral".
People used the Bible, including the New Testament, to justify slavery until the 1800s. It's amazing that no one noticed that it made slavery immoral until after society started to have ethical problems with slavery.
Springheel, I know for a fact that you're much more thoughtful than that. "The bible condones slavery" was already an unintelligible dead horse argument in the 19th century. And surely you recognize the several logical beggings of the question in your bald assertion (if I may syllogize it thusly:)
"people have used the biblical text to justify slavery; slavery is ethically unacceptable; therefore I need not pay thoughtful attention to what the historic biblical text or historic Christianity actually has to say regarding it".
Which people, and where, with what arguments? Which texts from the New Testament? What kind of slavery? Christian Thought has an ethical basis upon which to call slavery immoral -- Don't you need one of your own in order to say the same? And even if most Christians had supported slavery (and they never did), we surely don't make consensus gentium a test for truth?
Slavery as a metaphor in New Testament theological discourse is common knowledge. Paul, being a good educated 1st century man, used several legal metaphors to talk about Soteriology and the relationship of man to God -- slaves, sons, citizens, property managers, and more. That is the purpose of slavery in the places where it acts as metaphor.
You asked about the text from Ephesians 6 including, "masters, treat your slaves in the same way [as you would Christ]." If you are trying to pull that paragraph out of its meaningful literary context, in order to think of it as a bare standalone ethical maxim, you'll be sorely disappointed. If you're looking for veiled calls to social revolution, you'll be equally disappointed. Consider that the exhortations in the second half of Ephesians have an established foundation -- Paul isn't beating the church in Ephasus with a philosophically groundless moral bludgeon. There is a Christological and Soteriological premise for his cohortatives toward Christian living. Hence Anderson's argument that "the general idea of the texts is to recognize the brother in every servant and sort of to subtly suggest society to change its attitude." Even the very short letter of Paul to Philemon the slaveowner follows this pattern (in short: "Before you even consider what to do with this runaway slave, who is your brother in Christ, consider first how God in Christ has dealt with you.")
One of the only philosophical stances against slavery in the whole modern era (outside of Christianity) is Kant's Categorical Imperative and Enlightenment natural law theory. Two historically laughable disappointments with regard to abolition. Even the anti-Christian Marquis de Condorcet had to begrudgingly admit that "only a few philosophes have from time to time dared raise a cry in favour of humanity [over against slaveholding.]" Enlightenment natural law theory died when 19th century men discovered that there were hundreds of non-Western nations that didn't conform with the "rationality" of Enlightenment Europeans. And even if someone could demonstrate the correctness of either of these ethical systems, will people even follow it when it goes against their self-interest or value system? Thomas Jefferson is a perfect example -- he gave lip service to slavery being a "hideous evil", but he still remained a lifelong slaveowner, never attempted to stop his financial dependence on slavery, remained racist not only to people of color but also antisemite, and he made slavery a constitutional right in the expanding territories.
Meanwhile in Christianity, all classic Christians have always held that "if anyone is in Christ, the new creation has come," and the transformed Christian's value system and worldview will have soteriological motivation to treat his neighbor as himself.
Finally, let's bring some empirical and historical force to the arguement. Figures like Granville Sharp and William Wilburforce should need no explication, but permit me to quote a few other thinkers on this subject:
S. Scott Bartchy, “Slavery,” International Standard Bible Encyclopedia:
In such an economic context [that of the Roman Empire] it was virtually impossible for anyone to conceive of abolishing slavery as a legal-economic institution. To have turned all the slaves into free day laborers would have been to create an economy in which those at the bottom would have suffered even more insecurity and potential poverty than before. To be sure, according to all known traditions, neither Jesus nor His immediate followers owned slaves; nor did Paul, Barnabas, or Timothy. So both the example of Jesus and His great concern for the poor proved to be a challenge for many early Christians to conceive of themselves as living already among themselves in an alternative social-legal environment (note how Paul appeals to Philemon to release Onesimus sooner than he may have planned). For the author of 1 Clem. 55:2 Christ's love working through humble spirits has motivated some Christians to sell themselves in order to have money to buy the freedom of others (see Shep. Henn. Mand. 8:10; Sim. 1:8; Ign. Polyc.4:3).
Henri Wallon: Histoire de l’esclavage dans l’Antiquité (Written 1847--and already writing on the beneficial historic Christian impact on slavery in antiquity!)
The master had to spare the slaves as his equals in freedom; He was to treat them still more as being their brother in bondage; ... We are all born in bondage, we are all redeemed in Jesus Christ. (Nous sommes tous nés en servitude, nous sommes tous rachetés en Jesus-Christ.)
Thus, from the moment when Christianity had revealed its doctrine, the cause of liberty had vanquished. The day of triumph was to be delayed, it is true; And already the sign of salvation prevailed in the world, which was still awaited. But during these forced delays the Church did not forget the slaves; And at the same time preparing for them resources now honorable after emancipation, she pretended to give them a place in the domestic hearth, in the education of the family, in public esteem; It demanded for them all the rights and treatments of the free man
Alvin J. Schmidt, Under the Influence: How Christianity Transformed Civilization (2001):
It is an unarguable historical fact that the abolition of slavery in modern times stems directly from Christian influence.
Philadelphian Quaker Benjamin Lay, All Slave-keepers that Keep the Innocent in Bondage: (1737):
As God gave his only begotten Son, that whosoever believed in him might have everlasting Life; so the Devil gives his only begotten Child, the Merchandize of Slaves and Souls of Men, that whosoever believes and trades in it might have everlasting Damnation.
William Warburton, A Sermon Preached Before the Incorporated Society for the Propagation of the Gospel in Foreign Parts (1766):
“Gracious God! To talk (as in herds of Cattle) of Property in rational creatures!"
John Wesley, Thoughts upon Slavery (1774):
“The dreadful consequence of slavery is the same amongst every people and in every nation where it prevails. ... Thy [slaveowners'] hands, thy bed, thy furniture, thy house, thy lands are at present stained with blood."
Charles Elliott, Sinfulness of American Slavery: Proved from Its Evil Sources; Its Injustice; Its Wrongs; Its Contrariety to Many Scriptural Commands, Prohibitions, and Principles, and to the Christian Spirit (1850)
All men are redeemed by the same blood of Christ; and therefore, this common and general redemption by the blood of Christ is at variance with slavery. ... The same great sacrifice has been made for the slave as for the master; and therefore, the soul of the slave is worth as much as the soul of the master.