At this point a problem emerges; if Natural Law, which can be cogitated from pure reason, is not universal, is the pure reason which cogitated its discovery? One need not live in a society where adherence to natural law ethics was automatic or where everyone already always adhered to it, including the hypothetical subject. If that were the case, studying the problem would be a fabulous waste of time at best. Any study is primarily a building of one's own knowledge, and not for everyone else.

"It is a belief held by a subset of society -of which I consider myself a part - and is no more a universal truth than the concept of God to an atheist."

If an atheist asserts God does not exist, and God does exist, the atheist is wrong. If a theist asserts God does, and God does not, the theist is wrong. God does not come into existence, or cease to come into existence, at the individual's whim. Reality, in the metaphysical and ontological sense, is not subjective. The subjective nature of experience, including what one has taken time to think about, is still beholden to objective epistemological laws (logic) and these still come into effect no matter the experience or person.

As for the quasi-theistic presentation given in natural law arguments...most of the innovators in natural law theory were Christians, such as Aquinas, Locke, Jefferson, etc. and so it comes as no surprise to me that it should often have that framing, even though in its first exhaustive presentation, Aquinas, it was being deliberately separated from positive and divine law.

Edited a minor error...