Two features of moral life resist easy reduction.
The first is binding obligation. Most people, on reflection, hold that torturing a child for entertainment is not just distasteful or socially counterproductive but really, mind-independently wrong, even if every society came to approve of it. That is a claim about how things are, not about how we happen to feel. A standard form of the argument grants exactly this: there are objective moral facts, and then asks what best explains them.
The second is human worth. We treat persons as having a dignity that does not rise and fall with their usefulness, their power, or the consensus around them. The weak, the unborn, the enemy, and the prisoner each retain a worth that forbids certain treatment. But if a human being is finally just rearranged matter, it is hard to locate where that non-negotiable worth could come from. Christianity grounds it in persons being made in the image of God: a dignity conferred by the Creator, not earned by achievement or assigned by society.