Drinking battery acid can harm us, sure. But is drinking battery acid immoral? What if I’m trapped in a cycle of suffering and it’s the only way I can end it via suicide? Again, with slavery, some people are forced into servitude and often abused. But is forcing servitude and abusing immoral? What if my worldview happens to say that such and such races are not worthy of equal dignity? And that therefore enslaving such peoples is not immoral, but actually good and for the benefit of my own people. Who or what do you turn to in order to say that my worldview and the resulting actions is immoral?
So yes, we can say that suffering objectively causes harm. But we can’t say that causing that harm in the first place is objectively immoral. That’s just your opinion, equal to any other. You haven’t really shown me how, objectively, independent of your opinion, that slavery is immoral. You can show me the effects of slavery, but outside of your opinion, you cannot show me that slavery is actually immoral.
Atheism relies on consequences, well-being, law, society and such, in order to determine whether or not something is immoral. But all of these are ultimately subjective and unsatisfying, because they’re not objective. I want to be able to say that slavery is actually wrong, objectively, factually, truly, and not just my mere opinion that it is wrong. And it seems this can only be done when meaning, purpose and morality exists in some way, shape or form outside of our minds. But we know by observing reality that only powerful minds like our human minds are capable of considering such things, or producing them in the form of thoughts and such. Non-mind doesn’t produce meaning, purpose and morality, so it isn’t a stretch to say God is necessary in order for these things to exist objectively.
Indeed, I either read or was told that all the early existentialists who were atheists said the same thing. No God equals no objective meaning, purpose and morality. You simply have to face the absurd. But you can only do so by being delusional and giving meaning, purpose and morality to life where there technically isn’t any.