Been oft debated on this forum before, but it largely comes down to what way yo'ure using the word "nation".
Are you using it in the lay person vernacular where "Nation" and "Country" are basically synonyms and mean the government of "The United States of America"? IE "We as a nation seek to protect the world form evil". From that stand point, the answer would be no. The "nation" in this case is a secular one, adhering to no particular religion.
Are you using it in the other lay person method of using it, where it's referencing the PEOPLE within the United States in a generalized sense. For example, how someone could say "Football is the nation's new passtime." They're not suggesting everyone in the nation likes or watches football, but rather that its the most popular thing of it's category by the citizens of this country. In this case, it would be reasonable to suggest we're a "Christian Nation", just like it's reasonable to suggest that Football is the "nation's sport".
However, as it relates to the second notion...it's basically an irrelevant one. The fact that the majority of people prefer Football to any other sport doesn't mean the government should somehow reflect that in it's laws. Why? Because regardless of what the majority feel, it's reflecting upon what is essentially a personal opinion/choice of individuals and not a tennet of government. Additionally, notions like this are fickle and able to change through natural means different than the "government" version of the word nation. For example, at one time Football wasn't the "nation's sport", it was baseball.
Finally, the question would be are you using it in the political science sense, refering to people who share a common language, culture, ethnicity, etc? In that case, it could be argued that there is a large "Christian Nation" (specifically an american one) within the United States. The more accurate way to ask the question, as it relates to peoples normal meaning with it, would be if the USA is a Christian Nation-State. And that's a tricky one, as it depends on ones personal opinion of how much of those within a particular state need to share the common threads necessary to be a nation.
As it relates to all three of the above...
From a governmental sense, no we are most certainly not a "Christian Nation". Our government is secular.
From the "people of the United States" mindset, I think it was absolutely reasonable to suggest America was a "Christian Nation" at the time of the founding and some time after. I still think it's reasonable to possibly make the claim today, but it's a far more tenuous claim as the fervency of the majority is significantly decreased imho. IE, in the past I'd suggest that FAR MORE of those who identified as "Christian" were actively and routinely practicing the religion where as I believe many now consider themselves "Christian" but rarely attend church/actively engage in their faith.
As it relates to the notion of a "Nation-State", I'm believe that it would be reasonable to suggest in the early days that the United States was a Christian Nation-State. However, I believe it has not been for quite some time as the dominance of a singular religion in the majority has decreased to a significant enough degree in my opinoin, and the prevelence of that religion in the lives of those who follow it has also diminished.