Refuting Christians with their Own Bible

[GrandizerII]

The Bible just simply isn't explicitly clear on slavery being wrong. But putting that aside, it's not like we even have clear unambiguous access to all of God's moral rules, assuming God exists. So even if divine morality exists, there is no evidence that it is fully and readily available and that Christians or others are going by the rules of such morality rather than by the same or similar standards everyone else is going by.

If divine morals exist, why wouldn't we know them through the teachings of Jesus?

Because they're often vague and unclear, and don't sufficiently cover everything to do with morals.