Christianity has its own cultural-relativism problem. Biblical morality permitted slavery and polygamy; later Christians called those wrong. If moral progress needs a standard above culture, wasn’t the Bible’s culture measured and found wanting too?

This is the objection a fair reader raises immediately, and it deserves a straight answer. The Christian response is that the Bible itself contains the standard by which those practices were later judged (humans made in God’s image, love of neighbor, neither slave nor free), and that the trajectory of Scripture runs from regulating existing institutions toward undermining them. Critics reply that this reads later conclusions back into the text, and that debate is real. Either way, the argument for a standard above culture is not weakened by the fact that Christians have had to be corrected by it. That is what a standard above culture is for.