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A slave auction in Virginia is depicted in this illustration dated 1861.

By today's standards, it's hard to understand why the Bible accepted human slavery. The usual explanation: However sinful, it was too ingrained in ancient societies to eliminate.

An equally important question: Why did Bible-believers distinctly campaign against slavery? Abolitionism was "unique to Western Christianity. It certainly didn't exist in the Islamic world, where legal slavery existed until 1981 and where informal slavery still exists," evangelist Charles Colson writes.

Biblically inspired moral opposition has been portrayed in books by Lamin Sanneh of Yale University ("Abolitionists Abroad," Harvard, 2000) and Rodney Stark of Baylor University ("For the Glory of God," Princeton, 2003).

Sanneh says Arabs operated as slave-traders in Africa seven centuries before Europeans did. Before that, Christianization had largely eliminated the ancient practice in Europe.

Black evangelicals from America and Britain moved into Africa and abolished the internal slave trade, as Sanneh recounts, during the years when white evangelical activists in the West were ending trans-Atlantic trading.

The evangelical leadership raises two questions: Why did 19th-century evangelicals press "liberal" causes while today's evangelicals often promote "conservative" politics? And what about Roman Catholics?

The second question is partly explained by the fact that Catholics were a repressed minority in Britain and America during the abolition era.

The Catholic magazine Crisis addresses the matter further in an article by Ohio University historian T. David Curp in its September 2005 issue. Though some Catholic writers have sought to defend the performance of the papacy and church, Curp is fairly critical.

Some church decrees opposed slavery, Curp writes, but the popes endorsed Portuguese and then Spanish slave-taking "out of cruel necessity." That necessity, he explains, was that European Christianity was scrambling for survival against Muslim invaders who had enslaved "hundreds of thousands of Christians."

Churchmen used "every available means" of defense, including complicity with slavers. Though historical circumstances explain the papacy's performance "it's a scandal nonetheless," Curp says — interesting from a conservative magazine.

Curp also pins some blame on the Bible, saying Old Testament society was "as dependent upon slave labor as any other in the ancient world," and neither Jesus nor Paul demanded abolition.

Nineteenth-century Christians vigorously debated how to interpret these biblical materials. The Old Testament passages received detailed examination in "The Bible Against Slavery" (1837), a book whose text is available on the Internet. Author Theodore Weld, who experienced a dramatic evangelical conversion, became the abolitionists' chief lobbyist and strategist.

Weld's argument was in the first place theological, saying the Ten Commandments ("you shall not steal") oppose stealing a person's very self and that Genesis teaches each person is created in the image of God, not to mention scriptural themes of human freedom in Joseph's kidnapping or the liberation of Israel from bondage in Egypt.

Weld further contended that biblical slavery was far different from the chattel system in the American South. Most important, it was essentially a voluntary system of indentured service whereby needy people could enter six-year arrangements (see Exodus 21:2 and Deuteronomy 15:12-18) and obtain employment, room and board. Biblical law regulated the prerogatives of slaves and even demanded freedom if masters were abusive (Exodus 21:26-7).

The New Testament depicts only household slavery, which generally involved affection and virtual family membership. Interpreters think Paul implicitly appealed for a slave's emancipation in the epistle to Philemon.

The tiny Christian minority, of course, had no power to overturn such a universal economic system. Instead, Paul radically opposed distinctions of social status, which gradually had its own revolutionary impact: "There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus" (Galatians 3:28).