Natural law theory does not rely upon the existence of God or the non existence of God. It fits just as well into a Darwinian Universe and it replaced a political philosophy which did rely upon God, to wit, the Devine right of Kings.
The problem is that many people who denigrate natural law theory do not understand natural law theory and certainly have never read Hobbes' Leviathan or Locke's 2nd Treatise, let alone Grotious or Pufendorf or Rousseau. It is especially disheartening to see liberals trash these ideas, since they form they very roots of liberal political thought.
Natural Law theories demand certain precepts about how man developed and evolved. Most of the precepts Ive seen put forth are debunked by advances in science and understanding since the days of those you adore and put on pedestals. They all have clay feet
Being a liberal, I do not trash ideas so much as I move on from outdated ones.
Natural law theory states that anything we could do in nature is a "natural right". When we come together to form a society we form a social compact whereby we give up certain natural rights to enjoy the protections which society offers.
However, there are certain things which can not be bargained away in the social compact, because we would not be able to obey any law forbidding them even if whe tried... thus these things are called unalienable natural rights and it relies upon the fact that there are things that make us what we are. We need to breathe, we need to eat, we think, etc.
A law which tried to cut down on greenhouse emissions by making it unlawfull to breathe on Tuesdays and Thursdays not only would not be obeyed, it could not be obeyed, even if you wanted to.
People are thinking creatures and will believe what they believe. If a law was passed which required you to believe that the sky is lime gree in color, you might pretend it is, but deep down you will still believe the sky is blue.
Under this line of thought, unalienable rights are those biological and instinctual behaviors which define what we are... and as I taught my students... a bear has a natural right to shit in the woods. One problem area for many people is that unalienable natural rights can and are violated by governments. That does not mean that they are not unalienable... it just means that the governement violated your natural rights. Hitler violated the Natural Rights of Jews, because he made it unlawful to be what they were. They could not change themselves to comply with the dictates of the law reagardless of how hard they tried.
From unalienable rights we proceed to fundamental rights or auxillary rights as Blackstone called them. These are closely related to unalienable rights and serve to protect preserve and enhance an underlying unalienable right. The 1st amend primarily protects auxillary rights which protects the underlying unalienable right of freedom of conscience. The 2nd amend primarily protects an underlying unalienble right to self defense...the basic insictual fight or flight response... if someone were to throw water in your face you will blink your eyes, even if a law said that you could not.