Measured by the number of published editions, the most popular Puritan writer in seventeenth-century England was Richard Baxter. The popularity means that Baxter's ideas must reflect generally accepted views, and in his "Christian Oeconomics" Baxter discussed slavery at some length. What he said left nothing to be desired in terms of clarity -- I emphasize that calling somebody "incarnate devil" was pretty harsh language among pious early modern Englishmen. [The text comes from "The Practical Works of Richard Baxter," Ed. by William Orme. (London: James Duncan, 1830.) IV, pp. 212-220]:
" To go as pirates and catch up poor negroes or people of another land, that never forfeited life or liberty, and to make them slaves, and sell them, is one of the worst kinds of thievery in the world; and such persons are to be take for the common enemies of mankind; and they that buy them and use them as beasts, for their mere commodity, and betray, or destroy, or neglect their souls, are fitter to be called incarnate devils than Christians, though they be no Christians whom they so abuse."
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"Quest: But what if men buy Negores or other slaves . . . what must they do with them afterwards?"
"Answ: It is their heinous sin to buy them, unless it be in charity to deliver them. 2. Having done it, undoubtedly they are presently bound to deliver them: because by right the man is his own, and therefore no man else can have just title to him."
"Quest: But may I not sell him again and make my money of him, seeing I leave him but as I found him?"
"Answ: No; because you have taken possession of him, and a pretended propriety, then the injury that is done him is by you; which before was by another. And thought the wrong be no greater than the other did him, yet being now done by you, it is your sin."
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In general, there seems to be a correlation in history between the abolition of slavery and the more conservative forms of Christianity. For example, slavery disappeared from Western Europe between the eleventh and fourteenth centuries, and this disappearance seems to have been engineered by theologians and lawyers trained in the impeccably Catholic universities of the period. (As far as I know, modern Western Civilization is the only civilization in world history that has eliminated slavery.) An earlier example is provided by St. Augustine, who in the fourth century ran what could best be described as a one-man abolition movement in Hippo. Augustine appears to have had some effect in his hometown and its surroundings, but his effect did not last.
I am not quite sure about why conservative Christianity has regarded slavery as absolutely abhorrent. One possible explanation is the fact that no human must be in a position where somebody can force him/her to commit a sin -- this basic freedom of conscience was forcefully stated by Luther, when he defied both the Pope and the Emperor. A second possibility is a "ruboff" from the concept of "slavery to sin," which in conservative Christianity used to be one of the most disgusting imaginable conditions.