Is justice absolute or (culturally) relative? The Nazis† thought they were doing good: were they not? Were they "wrong" just because they lost WW2? If they had won, would they have written down that they were "right" and have history remember that 'fact'?
If they were morally wrong, regardless of winning or losing, where did that 'objective' moral fact come from?
You can certainly be an atheist and me moral and act justly. But where do you pin your morality to? If someone is an atheist, and probably a materialism (i.e., the only thing that exists is matter), where is justice in the scientific realm? What does a molecule of morality look like?
† Let's just invoke Godwin's Law and go for the extreme case.
And yes, there is a universe in which the Nazis won and it's taken for granted that they were right all along.
So now what?
Aristotle would disagree:
* https://tofspot.blogspot.com/2014/07/first-way-some-backgrou...
* https://tofspot.blogspot.com/2014/08/first-way-moving-tale.h...
* https://tofspot.blogspot.com/2014/09/first-way-part-ii-two-l...
* https://tofspot.blogspot.com/2014/10/first-way-part-iii-big-...
> The only absolute laws are the laws of physics.
Then you're left with explaining the laws of physics and why they exist. Those laws are contingent after all. See Aquinas's "argument from contingency" (though not the general cosmological argument or Kalām variant, which are not very good) as well as Leibniz:
* https://en.wikipedia.org/wiki/Cosmological_argument#Argument...
* https://en.wikipedia.org/wiki/Principle_of_sufficient_reason
For a fuller treatment see Feser:
* https://www.amazon.com/Five-Proofs-Existence-Edward-Feser/dp...
And if our sense of justice does come from God, why does it develop over time? Modern people who read the whole Bible are usually struck by how unjust the God therein is.