Assume for a moment, there is no soul and that a person is only physical entity. it follows that there is no "natural law" that can exist independent of a given culture. The fact that natural law has its origins in western philosophy, means that in order for it to become universal- it had to express itself
physically by the actions of men through the course of human history. So How does a western idea become universal?
By Force.
"Humanism" is part of the West's "civilising mission" to re-make humanity in it's own image. To conquer and annilate all cultures that do not conform with this "natural law" so that western culture becomes universal. What does not fit with "human nature" is "unnatural" and "inhuman" and not worthy of existence because it is not "moral". The physical annihiliation of said cultures may contradict the non-physical "laws of nature" but that does not prevent them from happening when the West is convinced of it's "humanity", that we are the "good guys" or when "god is on our side". The "non-physical" morality of natural law over-rides the physical existence of it's opponents.
In order to defend "human rights" or to assert "freedom and democracy" around the world, we wage endless wars to make sure that everyone else complies with
our concept of morality. This is true of the 19th century and remains true in the 21st century, because the "morality" rests on the assertion of a non-physical entity and therefore has no obligation to correspond to what we
actually do. This is how, no matter how many genocides, wars, persecutions, dictatorships, human rights abuses we commit, we always are the "good guys" because what is "good" has nothing to do with the physical realities of life on earth, or our actions, or their consequences. Natural law is God, and we can worship our "humanity" because it is the image of our perfection irrespective of who we actually are and what we do.
Humanism is hypocrisy; it is not secular, it's not moral, and it is most definetely not universal.