Second, for Christians and Jews at least, there is one devastating argument against them that I haven't seen refuted yet. Before the Israelites enter Israel, before they have a nation or an economy to even speak of, God gives the laws about slavery on Mt Sinai along with the Ten Commandments and all the other laws. Bada-bam. God instituted slavery. This argument works on Muslims as well. If not the revelation at Mt Sinai, then earlier. Why was there an institution of slavery in the first place? Why didn't God stamp it out in the beginning, such as with Noah onwards?
There are other arguments that religious believers may throw out, like how slavery wasn't the chattel slavery that Americans practiced, as if that makes things better. They may argue that emancipation was encouraged. That's explicitly true for Islam, but the very fact that manumission was encouraged for slaves implies that slaves existed in the first place for manumission. Islam and the other two religions don't challenge slavery. They assume it exists, the de facto state of things. And who's to say it wasn't the natural way of things? If God didn't like it, he would have commanded people not to take slaves. These were exactly the same arguments used by Confederate preachers, who pointed to the Bible as a justification for slavery. Not just Christians either. Here's an excerpt from a sermon made by Rabbi Morris J. Raphall in 1861:
He also comments on Christianity:How dare you, in the face of the sanction and protection afforded to slave property in the Ten Commandments—how dare you denounce slaveholding as a sin? When you remember that Abraham, Isaac, Jacob, Job—the men with whom the Almighty conversed, with whose names he emphatically connects his own most holy name, and to whom He vouchsafed to give the character of "perfect, upright, fearing G-d and eschewing evil" (Job i. —that all these men were slaveholders, does it not strike you that you are guilty of something very little short of blasphemy? And if you answer me, "Oh, in their time slaveholding was lawful, but now it has become a sin," I in my turn ask you, "When and by what authority you draw the line?" Tell us the precise time when slaveholding ceased to be permitted, and became sinful?"
Rabbi Raphall personally opposed slavery, but thought the Bible allowed it. If you're interested, there was a rebuttal from another Rabbi, by the name of David Einhorn. I highly recommend it, if anything because his acerbic takedown is hilarious.The New Testament nowhere, directly or indirectly, condemns slaveholding, which, indeed, is proved by the universal practice of all Christian nations during many centuries. Receiving slavery as one of the conditions of society, the New Testament nowhere interferes with or contradicts the slave code of Moses; it even preserves a letter written by one of the most eminent Christian teachers to a slaveowner on sending back to him his runaway slave.
And Raphall is right. You'd expect Jesus to overturn the nasty old religion with his pink and fluffy preaching. But he doesn't say that slavery is an immoral institution, and in fact Paul has a couple of rather gut-churning verses about slaves. Christians kept slaves for centuries. What changed between the Christianity of the 8th century and the Christianity of the 19th century? Were the preachers reading different scriptures from the pulpit? Were the audience somehow deaf to the 'pink and fluffy' Christianity for centuries?
I then get this argument that Christianity and Islam are progressive religions. Jesus wasn't a social revolutionary, he could've taken up arms against the Romans but he didn't. No, what he did was change the hearts of men so that eventually, they'd overthrow slavery by themselves. Then they mention William Wilberforce, but fail to mention that it was about two millenia too late. You get the same argment from Muslims, except it's rather hypocritical because they say that their religion is the final and most perfect revelation. Evidently not, if Muslims in the late 19th century had to abolish slavery.
Oh sure, this 'reforming of hearts and minds' did take place. But it took far too long and lead to enormous suffering and degradation for tens of millions of people. If he's not outright evil, they've just made God out to be callous and incompetent. Maybe that's not such a bad description.