Slavery Christianity's Untenable Arguement
The prescription for the practice of of slavery is one that directly conflicts the assertion of an objective authoritative arbiter for a standard of absolute moral perfection.
Arguements that Biblical slavery is distinct and radically progressive by comparison to race based chattel, does not make the moral foundation right it only argues biblical slavery, "indentured servitude", is less wrong ignores the Biblical prescribtions for the practices & ownership of foreign slaves, captives of war, and any children born to a slave wife provided by the master. This arguement doesn't not withstand moderate scrutiny as the source Biblical texts prescribe practices as permissible which are largely and undivisively deemed immoral.
Arguements for Progressive Revelation framework fail to address contradiction created when God the arbiter for a standard of an absolute perfect objective moral standard makes concessions on the basis of the morally subjective norms/practices of the times. Such arguements can only demonstrate a framework in the scriptures through which subjective interpretation could evolve towards general societal acceptance that slavery is immoral. Concessions made permitting subjectively "moral" practices, even to gradually reform, make it impossible for the arbiter to uphold the standards of absolute perfection regarding objective morality.
I would argue the only position one could hold without an inherent contradiction, is that slavery is objectively moral, an arguement which can only lose ground until it retreats to circular argumentation that God permitted it so it must be moral because God permitted it.