The United States of America

On what date was the United States formed? Options include:

• June 21, 1788, - ratification of the Constitution by the 9th state (NH)
• September 17, 1787 - completion and adoption of the Constitution by the Constitutional Convention
• March 1, 1781 - ratification of the Articles of Confederation
• November 15, 1777 - adoption of the Articles of Confederation by the Second Continental Congress
• July 4, 1776 - adoption of the Declaration of Independence by the Second Continental Congress

Abraham Lincoln seems to take the earliest date. Writing in 1863, he says "Four score and seven years ago our fathers brought forth on this continent, a new nation...." And 1863-(4*20+7)=1776.

A second question is whether this is trivial. I've heard some assume that it is not. Richard Land (in a class at SWBTS) has argued that the USA is theistic, since one of its key founding documents (namely the Declaration of Independence) is theistic. He brings in Lincoln's Gettysburg address as an authority on this. Others argue the USA is not theistic, noting that the Constitution is a secular document. The former use 1776 for the birth of the nation; the latter seem to be committed to either September 1787 or June 1788 as the nation's official beginning. Can an argument be defended either way on this point? How is it that nations are formed, anyways?