So this begs the question; why is natural law your supreme authority? You may say that once you point towards natural law, you have left the realm of philosophy; but you haven't. I believe in God as my supreme authority, and from Him I derive all my beliefs. You point to natural law (or science) as your ultimate authority.
There is evidence for one, but not for the other.
All the world's religions rely on belief, dogma, scripture, divine revelation, and other non-empirical and non-verifiable forms of "evidence."
Science relies on empirical evidence, that is, things that can be measured, quantified, or directly inferred in some way.
And if, as you said, the morals are subjective, how can the law justly punish someone who has committed a moral wrong if that particular person believes that the crime they committed was actually a good thing under their system of morals.
Morals may be subjective, but laws are not. If you behead an "errant" daughter in this country you violate a law no matter what your personal delusions may be.
Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.