The "right to life" is seen in the objection to a government killing a citizen for no reason. The argument is that, self-evidently, it's wrong for a government to do that. So there must be a right to life independent of the laws of a country or the whims of its rulers. Sure, a theist would claim that God would in particular object to capricious killing, but you don't have to be a theist to claim it's self-evidently objectionable.

And I guess you can argue that some do not recognize universal rights, but it's not a compelling argument to just point out what some people think.

Not at all. The concept of natural rights merely means that by our nature we have rights. The schools of thought that led to that identification and to the foundation of the US based on the ideal of a government exclusively tasked with securing these rights goes back at least to the 15th century (Aristotle and other Greek thinkers already were inching towards the idea that there are things that are right to protect in the polis despite what others may think or the laws may impose; for example, read Antigone by Sophocles).

What is it in human nature that concretely and morally entails our having rights? the faculty of reason and its crucial role in human life, as long as we are free to act according to our thoughts. No faith involved, no mysticism required.

[added] The Founding Fathers did not create a theocracy. I don't see what point you are trying to make -- the concept of natural rights does not require a god, and it is irrelevant whether some person invoked deism or not. The fact is that we are all born with inalienable rights by virtue of our nature, what we are and how we ought to live together.

> The concept of natural rights merely means that by our nature we have rights.

The Greeks may have had a concept of natural rights separate from their being endowed by a Creator, but that is not the same concept of natural rights that John Locke proposed and which inspired the American Founding Fathers, leading to posts from Americans like the one we see above. Advocacy for that school repeatedly cites the existence of a Creator.

John Locke and many American Founding Fathers definitely do cite the existence of a Creator, but that isn't a fundamental requirement of the argument. Part of the derivations of natural rights (at least in Locke) comes from observing animals in nature, and applying these "natural" rules to humanity.

The french rights pronounced by the French Revolution had nothing to do with the idea of a Creator and were very similar in concept with many of the Founding Fathers'. So this does not seem like a reasonable argument to make.