Christianity has no rules on how to treat slaves other than one or two sentences in the epistles of Paul telling people to be kind, and another encouraging freeing slaves.

This is far form endorsing slavery. it was regarded as a necessity for society to function at the time. It is rather like how Christians now often accept the accumulation of wealth that is inevitable in a capitalist society as better than the alternatives. St Paul also said slaves (and women, and people of different races) were equal before God ( Galatians 3:28 ).

The Bible has rules about how to treat slaves, but from a Christian point of view these did not apply - they are Jewish law similar to not eating pork or requiring circumcision etc.

Christianity was the main motivation behind abolishing slavery and we have written records of Christians calling for slavery to be abolished going back to the 4th century.