Unlike most nation-states, what makes Americans a unique national entity, what provides our identity is not ethnicity, or language, certainly not religion. What makes an American, well, an American is that remarkable document that created our current form of governing structure, both reflects the changes of the past two centuries as well has helped effect some of those same changes, and remains, despite all the criticisms and claims to the contrary, the living, breathing heart of this country.
Our military does not make us free, the Constitution does. Our police do not keep us safe, the Constitution does. Our industrial might, our technological superiority, our vast entertainment industry do not make us the envy of the world, the Constitution does. Our collective acquiescence to any particular economic practice does not provide opportunity, our Constitution does.
To be an American is to live under the Constitution, to admire its flexibility as well as its intransigence. To be an American is to abide by the idea that we are a nation not of race or religion or language but a nation of laws, binding on all. One can be a communist, an atheist, a Catholic, a Lutheran, a libertarian, and still be fully American. One can speak Spanish and Armenian and Urdu and Tagalog, and still be an American. The single requirement for being an American is swearing allegiance not to any God or economic or social system, but to the Constitution. The military and elected officials do not swear an oath to capitalism or plutocracy but to the Constitution. From the President to that homeless woman on the corner, ideally all are bound to the limits and strictures not of power or privilege, but to the Constitution. Making these ideas and ideals real in our day-to-day living is what it means to live as Americans.
If I am ever asked what I think it "means" to be an American, this is my answer. We are not a culture, let alone a single society bound by shared history and traditions outside the civil and legal ones enshrined in the Constitution. The Constitution knows no preference regarding gender or race, religious or moral code, but sees all as part of what makes the United States the truly great and unique place that it is. To be great is not to be militarily unchallenged, still less economically powerful. To be great as America is to celebrate our differences bound only to the civil practices set forth in the Constitution. This is the source of our strength, because it is, in the most fundamental way, who we are.