I thought Thomas Jefferson gave us that.
And who's understanding and definition of the word 'God' legally controls in this situation? Upon what grounds? I do not see a unified acceptance of what anyone might mean when using that word. Nor does it acknowledge the plurality of its use. Invoking that word solves nothing. Political compromise is still required. Although some wish that it were, the United States is not a theocracy.
Thomas Jefferson was not expressing an idea that had historically been considered self-evident and true. It is instead, an expression of an imagined better state of affairs, and a declaration that he and his fellow colonists intended to make that imagined state a reality.
As has always been the case, it is we human beings that have been making up the rules as we go along.