What is Christianity's greatest moral advance?

The greatest moral advance Christianity introduced is this: that every human being has equal, inviolable moral worth grounded in what they are, not in what they do, contribute, or survive.

Not strength.

Not usefulness.

Not tribe.

Not intelligence.

Not evolutionary advantage.

But being human.

That claim did not exist in the ancient world as a moral axiom. Ancient ethics were hierarchical by default: worth scaled with status, power, sex, ethnicity, or utility. Christianity shattered that logic by grounding dignity in the image of God, not in social function.

That move does something no evolutionary or pragmatic ethic can do:

• It makes morality obligatory, not merely advantageous

• It protects the weak even when they are a burden

• It condemns exploitation even when it benefits the group

• It makes injustice wrong even when it works

This is why Christianity could condemn slavery in principle, even when societies still practiced it. Gregory of Nazianzus didn’t argue that slavery was inefficient or outdated. He argued it was ontologically false — an assault on God’s image.

And this is the key point your critic keeps missing:

Evolutionary “morality” can explain why we feel empathy.It cannot explain why we ought to obey it when it costs us. Christianity supplied that missing “ought.”

That single move — grounding moral obligation in the nature of persons rather than in survival outcomes — is the moral engine behind human rights, abolition, care for the disabled, the equal dignity of women, and the protection of the unborn and the dying.

Secular humanism later inherited these conclusions, but not the foundation that made them binding. So if I had to name one moral advance Christianity introduced, it’s this:

Every human person has within them a non-negotiable value, dignity and worth that surpasses their usefulness, allegiance, tribal affiliation, or any other secondary attribute.

Morality is built on this. Everything else flows from that.