Both the Declaration of Independence and the Northwest Ordinance were composed prior to the ratification of the US Constitution. The Founding Fathers composed the US Constitution not as a document that is more secular; they composed one that is entirely secular. The Founding Fathers could have composed a document that was sectarian; as James Madison's Convention notes reveal, the subject was never considered.
The US Constitution was composed and ratified in reaction to the ineffectiveness of the Articles of Confederation as a national governing fundamental law. The states that emerged from the territory governed by the Northwest Ordinance composed constitutions that comported with the US Constitution.
That doesn't mean that the Constitution is based on positive law. The Constitution is derived from the Virginia Constitution, which is also secular, but both assumed natural law, much of the through John Locke. Considering that all of the authors were theists, considering their education and influences, and considering their own writings, there is no doubt that the Constitution is a natural law document up to and including the first ten amendments -- and probably several subsequent ones too. Remember, nonsectarian does not mean atheistic nor even agnostic. That means it's not "totally secular" as you assert. I recommend reading David Sehat, "The Myth of American Religious Freedom" (Oxford University Press, 2010) for historical context.
