Given that as you say, the majority of the population is Christian, it certainly does make the USA a Christian nation.
If we follow your logic, does it mean that Israel is a Buddhist country?
Do you see how illogical your statement is?
But one could almost argue that there are policies, condoned and enforced by various levels of government, that are designed to eliminate religion, particularly Christians, as they are the easiest target and the most docile.
A great example is school districts having winter holiday celebrations as opposed to a Christmas Party, but at the same time not censoring the names of other religious festivals (Hanukkah, Ramadan, Diwali, etc.). Why not call it what it is?
Just my $0.02