One of the most important points made by Jefferson in the opening of the Declaration of Independence, one that is easy to overlook given what follows the first paragraph, is Jefferson's argument that one group of people has the right to separate from another group of people
and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them.
Jefferson's reference to two seemingly separate legal authorities here—Nature and Nature's God—have been the subject of intense speculation about whether Jefferson refers to two sources of laws or conflates Nature and God into a single entity. We know, based on Jefferson's writings, that he is skeptical of many aspects of biblical tradition and Christian doctrine—after all, he lives in and exemplifies the Age of Reason—but it is reasonable to conclude that his argument here is a rhetorical device to express the overwhelming legal justification for the separation. Syntactically, the two are separate and equal, but philosophically, they are one and the same. In short, Nature and God, although mentioned separately, are one and the same, and the laws of this collective entity justify the separation.
Another very important argument in the Declaration, existentially threatening to George III and other European monarchs, is Jefferson's belief that
... Governments are instituted among Men, deriving their just powers from the consent of the governed, --That whenever any Form of Government becomes destructive of these ends, it is the Right of the People to alter or to abolish it, and to institute new Government.
This argument shocked the monarchies of Europe because it relies on the premise that "just powers," not the divine rights of monarchs, form the basis for government. Even though Great Britain has a limited monarchy at this time, George III is still a powerful monarch in theory and in practice, and Jefferson's argument that the governed have a right, based on the exercise of unjust powers, to create a new government threatens the system that provides a monarch his or her power. In fact, Jefferson establishes a new paradigm for government rule that, in effect, obviates the monarchy.
After noting the many appeals the Americans have made to both Parliament and the King to redress what the Americans view as oppressive measures to punish and subjugate America, Jefferson places the blame fully on the titular head of Great Britain:
A Prince whose character is thus marked by every act which may define a Tyrant, is unfit to be the ruler of a free people.
Consistent with his earlier argument that a people has the right to institute its own government, should it suffer oppression from the present government, and mincing no words, Jefferson converts George III, who has abrogated his responsibility to govern justly, from prince to tyrant and pronounces him "unfit" to govern.