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Is Christianity anti-slavery?
It was, of course, Christians that originated the abolition movement. Specifically in England - first Quakers, then evangelical Anglicans. Without Christianity (and the British Empire), slavery would still be accepted and widespread.
Yet, of course, since abolition did not arise until around 1800, so, for a reactionary the question answers itself: Christianity is not intrinsically against slavery.
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Slavery was an almost universal feature of settled societies, and until around 1800 in England nobody argued that slavery was intrinsically unjest (although nobody wanted to be a slave themselves, as an ideal; although it might be the best available option in some situations). There were, however, plenty of religious arguments that slaves ought to be well treated, if possible.
Of course, slavery is a conitinuum, which makes it hard to discuss and possible to deny. At one extreme lies the irreversible ownership of humans and the ability of the slave owner to do anything whatsover to the slave with no social sanction of any sort. On the way to that are forms of reversible slavery (buying or winning freedom), time-limited slavery (indenture, military conscription), or partial slavery (serfdom).
Certainly there is a wide range of how well slaves have been treated - in the Northern Americas slaves had families and the slave population grew while in the Southern Americas they were (at least initially) worked to death and needed continual replacement from Africa. There is little evidence left of the vast numbers of white slaves taken to the Middle East.
Secular regimes have slavery. Communism reintroduced slavery on a massive scale, but secretly and under other names and justifications; which meant that the slaves could be (and were) as harshly treated as any in world history. National Socialism also reintroduced slavery.
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But Christianity as such has little to say about slavery, or indeed any other political or economic arrangements. Most people in most societies through history have been extremely poor, worked long hours and were semi-starving for 'Malthusian' reasons; under such circumstances when there is not enough, clearly Christianity cannot insist on any minimum standard of living or comfort for slaves - or, indeed, for anyone else.
(Hence, all attempts to link Christianity with specific economics are mistaken.)
Christianity (I think this is fair to say) is intrinsically in favour of 'decent' treatment for slaves: slaves ought to be treated as people having immortal souls and free will (and not as animals), and with decent treatment conceptualized such that slaves are able satisfactorily to practice their religion - to be a good Christian in accordance with how that is conceptualized.
Therefore Christian treatment of slaves is more about Time than Conditions: sufficient time must be be available for slaves (like everyone else) to engage in prayer, to participate in Church services and sacraments, the reading of scripture, festivals, fasts, feasts, pilgrimages - or whatever is deemed necessary to be a Christian.
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