This is one of those questions that is asked a lot but is never answered to everyone's satisfaction, isn't black & white, and will probably never really be answered.
I can only say that for myself, as a non-religious person (by choice, as is my husband), I see myself living in a society that certainly has many Christian-only relevancies. Christmas, for instance. No, we don't observe the religious aspect of it, but we certainly observe it, and it is certainly a Christian holiday. Same for Easter. I don't observe Rosh Hashannah, or Ramadan.
I don't believe in God, but I say "Oh my God" at least 2 times per day. I don't say "Oh my Allah".
So even though we don't intend to, it seems that most of us have "Christianity" as part of our lives.
The groups who came here in the 1600s came here for religious reasons. The Puritans couldn't observe their religion in England so they came here to be free. Of course that was an odd brand of Christianity and when they got here, they banned people from society for not observing their views, which is the same thing that happened to them in England, causing them to come here.
I think probably the founding Fathers assumed Christianity would be the prevailing religion in the USA not having much if any exposure to Hinduism and Muslim and other religions, but they probably didn't assume it would be exclusive.
So while that was all a non-answer, I'd say no, we're probably more a melting pot of religions, but yes, Christian and all its strains is the prominent one so if you base it on pure population alone, we would be a Christian nation. If you base on it the way it should be governed, then no, it shouldn't be considered a Christian nation.