That's just flat our wrong. Morality is fluid and has been re-defined by people living in tribes or larger societies. One can argue fairly well that we would already live in a more just world if we would have skipped that non-sense of believing in a higher being.

For example, we now agree slavery is kinda bad. Yet, not too long ago the bible was used to justify slavery around the world. Who's to thank for this progress? Certainly not religion.

The Christian religion (including the Bible, but not always by the subset of Christianity which holds that the former is entirely contained in the latter) was likewise used to justify abolition, and this, like justification of slavery, goes back to the fairly early days of organized Christianity.

That Christianity isn't a unified hive mind?

> Shouldn't the literal word of the most moral being in the universe be clear regarding slavery, at the very least?

Arguably it should be, but then the idea that the Bible is the literal word of God is not, itself, a doctrine shared by the whole, or even majority, of the Christian community (it's a doctrine primarily of the evangelical and fundamentalist branches of Protestantism.)

If they claim to have direct communication with the creator of the universe, like all christian communities do, they should have at least a minimum standard of congruence. Or is he just having a laugh with some of them?

Well (1) all Christian communities do not claim that all Christian communities have direct communication of that kind (in fact, Christian communities often claim that other such communities are deficient, and perhaps willfully so, in there regard), and (2) I don't even think all Christian communities claim that even they themwelves have communication of the kind that would support any conclusion much stronger than that that community’s authoritative teachings are free from error (certainly not that any subject of a specified degree of material impact should be addressed.)

Moral questions are (or at a minimum involve subordinate questions which are) outside the scope of empirical science; in this way they are similar to many religious questions (and the categories overlap), but they are not the same as, or a subset of, religious questions.

Yes, they are. Science can answer is, it can't answer ought.

(It can help reach practical conclusions from moral axioms applied to concrete facts, but that's not really answering moral questions: the choice of axioms is the thing that answers the moral questions, and science can't help you there.)