I already tried to semi-counter it, but all of that depends on your definition of authority and who has the authority to write a law.

Perfect example are the Neurenberg trials where many Nazi's defended their actions by saying that all they did was to follow the laws of their country. This argumentation in the end did not hold up, but why? Because the allied countries believed that there were laws that go above the laws of a country. These later became human rights and international laws on warcrimes, genoside and the like.

Weakness is again that it is humans determined humanrights, just as it were humans who wrote the laws for the third empire. Humans make laws, humans can rewrite laws.

I, in my answer, already pointed to a perfect lawgiver who would write a perfect law. Without such a perfect lawgiver there is basically no basis for judging laws written by humans other than our current (collective) emotional state.
Now I do believe there is a God and therefor in a perfect law(giver). If I would not I would have to completely rewrite and rethink everything I've just said and mabye conclude that justice is nothing more than an experience based upon genetic and cultural preferences.

The way justice is historically used, is somewhat in the way I described above (even though they were polytheists) and is the way in which it functions in our societies and lives. It is part of our way of life and our way of thinking and has been for thousands of years. It is part of what makes us human.

I would therefor say that to deny God is to deny humanity.

How on earth did I end up stating that? I guess I've been reading too much Lewis and Chesterton