“Natural Law” is
not based on religious belief. Another name for “Natural Law” is “Universal Law” and involves morality, not religion. It’s reflected as the final step in development of morality. It is because of Universal Law that abortion is wrong, that all people have basic rights including the right to life, food to eat, and freedom to develop autonomy (among many more).
Kohlberg, a psychologist, is usually given credit for developing the well-known stages of moral development.
"In Stage six (universal ethical principles driven), moral reasoning is based on abstract reasoning using universal ethical principles. Laws are valid only insofar as they are grounded in justice, and a commitment to justice carries with it an obligation to disobey unjust laws. Rights are unnecessary, as social contracts are not essential for deontic moral action. Decisions are not reached hypothetically in a conditional way but rather categorically in an absolute way, as in the philosophy of Immanuel Kant. This involves an individual imagining what they would do in another’s shoes, if they believed what that other person imagines to be true. The resulting consensus is the action taken. In this way action is never a means but always an end in itself; the individual acts because it is right, and not because it is instrumental, expected, legal, or previously agreed upon. Although Kohlberg insisted that stage six exists, he found it difficult to identify individuals who consistently operated at that level."
en.wikipedia.org/wiki/Kohlberg%27s_stages_of_moral_development (bolding added; yes I know this is from Wiki but I like what the site states and I have limited time here)
Those people who are religious may be more apt to reach the stage where they understand Universal Law, but the Law is not based on religion.
Kohlberg did theorize that in a seventh step religion became involved, but he found it very difficult to show this step in any sort of empirical way (he also had problems with stage six) and IMHO that is because so few people actually develop that far, which is a shame.
BTW, are you actually suggesting that only religious people can understand Universal Law? That atheists are too dumb to understand? That something that cannot be proven empirically doesn’t exist? First you say there is no way to demonstrate that what you call “Natural Law” exists. Then you say it is based on religious belief. Well, which is it? If you can’t prove it exists why are you stating what you believe it is based on?
Universal Law (Natural Law, Moral Law) is TRUTH. It does exist, independent of religious belief. "Depending on religious belief " smacks of moral relativism.