TWO hundred and fifteen years ago yesterday, the Continental Congress, meeting in Philadelphia, adopted the Declaration of Independence.That began a 13-year process of institutional innovation and nation building, which culminated in 1789 with the installation of a new government under a new Constitution for the United States of America. John Quincy Adams - son of the country's second president and himself its sixth - looked back on this extraordinary enterprise in a "Jubilee Discourse" on the Constitution, which he delivered in New York in 1839. The Declaration and the Constitution, the former president argued, "are parts of one consistent whole, founded upon one and the same theory of government, then new...." Both of these great constituent documents were proclaimed in the name and by the authority of a nation and its people: "When in the course of human events," the Declaration begins, "it becomes necessary for one people to dissolve the political bonds which have connected them with another...." Similarly, the Preamble to the Constitution proclaims that "we the People of the United States, in order to firm a more perfect Union...." Like most of the founding generation and like Abraham Lincoln after him, John Quincy Adams saw the Declaration as the ark of the American covenant. Acceptance of its ideals - natural equality, inalienable rights, and popular sovereignty - gave the nation its only certain form and sustenance. In 1839 Adams saw these ideals in retreat, in the face of "anti-revolutionary theory, which arrays state sovereignty against the constituent sovereignty of the people, and distorts the Constitution of the United States into a league of friendship between confederate corporations." Adams's great discourse is touched by a profound worry, as was Washington's Farewell Address some 40 years before it, that the centrifugal pressures of particularistic claims, especially assertions of states' rights, would prove too strong and sunder the nation built on the Declaration's ideals. The old patriot pleaded of his fellow citizens, paraphrasing Scripture: "Lay up these principles in your hearts, and in your souls - bind them for signs upon your hands, that they may be as frontlets between your eyes - teach them to your children, speaking of them when sitting in your houses, when walking by the way, when lying down and when rising up - write them upon the doorplates of your houses, and upon your gates - cling to them as to the issues of life...." The whole experience with slavery, which culminated in the Civil War, showed Adams's doubts and fears all too well-founded. But well-placed, too, was his underlying confidence: There was a national idea of enormous force and vitality. Its strength enabled the United States between 1881 and 1924 to take in as citizens and make its own some 26 million immigrants, most of them from other than English-speaking countries. The entire population of the United States in 1880 had been just 50 million. E Pluribus Unum was no empty boast. The power of this integrating idea has won the admiration of foreign observers, including some who were hardly fans of the United States system generally. In "The Future in America," British socialist H. G. Wells wrote of the wonder of a million immigrants pouring through the Port of New York in 1906 alone, finding jobs and becoming members of a new society. Just this past March, Margaret Thatcher, another Britisher, who is a fan of the United States, told a Washington audience that "no other nation has been built upon an idea - the idea of liberty. No other nation has so successfully combined people of different races and nations within a single culture." Never before have the benefits of America's integrating sense of nationhood, built on shared beliefs, not any particular ethnic background, been more evident than they are today. Many countries around the world, including the Soviet Union and Yugoslavia, are experiencing terrible strains because of a failure to offer their ethnically diverse people a convincing sense of belonging to some larger, more compelling nation. Even established democracies like France and Belgium face troubling divisions involving immigration flows that are extremely limited compared to what the United States received historically and receives today. What's more, the United States has made strides, in the area of race relations especially, in honoring more fully its founding ideals. And the appeal of those ideals commands the allegiance of today's immigrants as it has the loyalty of generations past. The philosophical basis of America's founding, as set forth in the Declaration, remains compelling today. The United States doesn't now face threats to its nationhood and unity at all comparable to those of times past - most notably, of course, in the Civil War era. But it is important nonetheless that we not take the basis of our unity for granted. The current infatuation in some circles with the teaching of "cultural diversity," emphasizing what makes our various ethnic strains different rather than the vital essentials that unite us, shows an unfortunate indifference to the source of our life and progress as a nation.