Yes and no.
There's two types of Nations technically speaking, the cultural and the political.
Without a doubt America was a Christian Nation at the founding and continues to be one to this day. At the time of the founding the vast majority of the population shared a common cultural bond, that being the religion they shared which was at its based Christianity. This particular cultural lynchpin is second only to language, as the most constitant common bond amongst the majority of individuals within our generalized borders. From the earliest days till now there have been a large variety of ethnic backgrounds. While sharing a common history, the immigrant nature of our country makes this a slightly less concrete connection as well.
Its reasonable to assume that likely close to 95%+ considered themselves some form of Christian at the time of the countries founding, with it still being more than 3/4ths of the country today considering itself as part of that faith. The overwhelming majority that represents Christians in this country makes it unmistakable to me that as a cultural nation the U.S. is without question a Christian Nation.
I answered Yes and No however, because as a political nation we are joined not by the notion of Christianity but by the notion of governance by the people, for the people, with an understanding of an inherent right to freedom for individuals. The Delcaration of Independence was written on the notion that a political nation can establish its own right to rule. In some ways one could potentially argue its due to this reason that the U.S. is a Nation-State, though I don't quite know if the term would be appropriate for it. This is not to say that in this case it is a "secular" nation, as while freedom of religion in general was a portion of the initial make up of the country that notion did come after the true founding of the American NATION, coming into being truly during the time of the Constitution. Additionally, while not directly relating to a specific religion, the notion of divinity or of some sort of higher power is present throughout the founding documents and the arguments for independence and the formation of said political nation, so arguing that it is a secular nation even on the grounds of a political nation to me would be wrong. It would be however an argument that America is a nation of freedom.
So to me, its two fold in how one could describe the U.S. as a nation.
In regards to culture the United States is a Christian Nation, while with regards to politics the United States is a Nation of Freedom.
Your thoughts?