"Civil Rights"? That is modernist version of natural rights, and natural rights was derived from 'natural law', a concept developed in the West first by Saint Thomas Aquinas, the middle ages theologian. That concept was carried forward by Locke and others, and then by the Americans who wrote the DOI.
In short, your real 'firm foundation' is Aquinas the Christian theologian and his 'natural law'.
See e.g.,
http://www.newadvent.org/summa/305701.htm and
http://www.newadvent.org/cathen/09076a.htm "According to St. Thomas, the natural law is "nothing else than the rational creature's participation in the eternal law" (I-II, Q. xciv). The eternal law is God's wisdom, inasmuch as it is the directive norm of all movement and action. When God willed to give existence to creatures, He willed to ordain and direct them to an end. In the case of inanimate things, this Divine direction is provided for in the nature which God has given to each; in them determinism reigns. Like all the rest of creation, man is destined by God to an end, and receives from Him a direction towards this end. This ordination is of a character in harmony with his free intelligent nature. In virtue of his intelligence and free will, man is master of his conduct. Unlike the things of the mere material world he can vary his action, act, or abstain from action, as he pleases. Yet he is not a lawless being in an ordered universe. In the very constitution of his nature, he too has a law laid down for him, reflecting that ordination and direction of all things, which is the eternal law. The rule, then, which God has prescribed for our conduct, is found in our nature itself. "
This view of Aquinas as our rights and obligation deriving from 'natural law' is reflected in the thinking of our Founders:
"A free people [claim] their rights as derived from the laws of nature, and not as the gift of their chief magistrate." --Thomas Jefferson: Rights of British America, 1774.
"The God who gave us life gave us liberty at the same time; the hand of force may destroy, but cannot disjoin them." --Thomas Jefferson: Rights of British America, 1774.
"Can the liberties of a nation be thought secure when we have removed their only firm basis, a conviction in the minds of the people that these liberties are of the gift of God?" --Thomas Jefferson: Notes on Virginia, 1782.
... and the DOI is not merely a catalog of complaints, but a testament to this philosophic belief in the Natural Rights of Man. It is thus vitally important that is was was "endowed by God with inalienable rights ..."
Law has several levels.
Physical law: Example-The speed of light is 186,284 miles a second, it is not just a good idea, it is the law. Physical laws take care of it self.
Natural Law: Example: People retain the right of revolution. Of course, Tyrants retain the right of oppression. The strongest wins. Self evident indeed!
Constitutional Law: The prescription of organizations, powers, and prohibitions associated with the founding documents of a nation. It can either be a single document, or a vast number accumulated over time as is done in the UK.
Legislative Law: Made by the legislature, and in the US approved by the executive (or the legislature overrides his veto). This includes rules on filibusters in the legislature itself.
Common law: The body of court cases which provide guidance to judges. Most of this covers procedures in the US since matters of fact are normally reserved to the Jury.
Tradition: habitual practice, which can be changed by any of the above. If physical law changes ( a new value of pi, due to changes in the physical universe) then the traditional value of pi is no longer used, because it doesnt work.
That all men are created equal is not self evident. We all have differences, and wealth, royalty, stature were all unequal. Oh but it sounded so GOOD, and was a mighty poke in the eye to the servants of the king! The D of I was a mighty work of propaganda! By objecting to it the King was cast in the role of tyrant or legal nitpicker, either of which loses.