Conscience and the Moral Argument as Evidence for the Goodness of God.

[Fake Messiah]

As I explained many times, from humanistic perspective slavery is bad because a humanistic perspective is based on individual human feelings, and we know that slaves suffer. In Christian societies, the individual is not important, so that is why slavery was often acceptable (and many Christian apologists want it to come back) as is not allowing women to be educated, tolerance of other religions is not allowed, etc.

But there is no objective way for me to say that slavery is bad. I can only appeal to logic and humanistic appeals, which can easily be dismissed by a theistic mentality.

If you can appeal to logic, then you literally -are- saying that slavery is objectively bad.  People might dismiss it, but people often dismiss facts.  That doesn't change anything about those facts, it just tells you about that person.

Logic but inside of the cultural frame. Let's say there is a society where slavery is moral because they believe that their god said it was moral. Now you could appeal to human feelings that slaves are suffering, but if they believe that their god is real then individuals don't matter and you are the one who is dismissing the facts. So the only thing you could do would be to debunk their god because to them it is logical to do what their god wants.