I want to address an underlying misperception is see here. I am not supporting any particular group or trying to establish what is right. I want to address basic realities. I am not going to address the justice or injustice of history here.
First, recognize that our labels are generalities and in some ways inaccurate, in others quite accurate.
I am going to say America is a Christian nation, but before you pull the torches and pitchforks, hear me out.
Everyone knows we have rich parts of town, poor parts of town, ethnic parts of town, and we accept and use these terms in everyday speech, even though we realize that not every resident in that part of town fits the generalization. Obama's statement that the USA is and always has been a Muslim country was typical political bullshit and, I think, was recognized as such by basically everyone.
I think everyone recognizes, for all practical purposes, Saudi Arabia is a Muslim country. I'm sure there are people who aren't muslim living there, but Islam (with some pre-Islamic influences) has had an overwhelming influence on the culture, laws and customs. Saudi Arabia enforces this in their laws. Islam is their official state religion.
India does not have an official religion. Realistically though, the majority of the population is Hindu, with relatively large Muslim and smaller Christian minorities. Of course this has effected their laws, customs and mores. There are probably many areas of India we could call Hindu, and some areas we could call Muslim without being too far off, even though these areas have small populations that don't fill that bill.
Prior to European settlement, the inhabitants of that land had various religious beliefs. When the European settlement happened, it came from countries where the populations were overwhelmingly (>90%) Christian. (I realize and accept the fact that their Christianity contained a lot of pagan elements, i.e. Christmas and Easter, but for lack of a better word, I will call their belief system and world view 'Christian' or maybe 'European Christian'). The areas they took over became 'Christian'. Maybe a better way to say it is 'europeanized'. The overwhelming majority of the people had a certain culture, even though there may have been minorities from different cultures living in the area. There were areas where european culture melded with to local cultures (Mexico) because the local population was largely not replaced. Other areas where essentially complete replacement occurred (Chicago, Boston, New York). I don't think it is out of line to say the areas where complete replacement happened became Christian. The melded cultures are a blend, but most would accept the 'Christian' label, although slanted a different direction.
Traditional american culture has some important influences from african and native american cultures, but the majority of american culture, for better or worse, is what we could call "european" of a variety most would call 'Christian'. This 'general culture' has been the basis of our laws, customs and mores. In this way I would say America is a 'Christian nation'. This isn't set in stone. It may change in the future due to immigration, shifting social mores, etc, but at this point I would say the USA is still a 'Christian nation", in the same way 9th century Norway and even early 10th century was Pagan, even though there may have been some christians living there.
Our constitution and political customs reject the idea of a 'national religion' or giving a single religion preference. I agree with that, partly because I have lived in areas where the overwhelming majority went to a certain church and I don't think it's healthy, either for the majority or minority, for different reasons. We sharpen up by rubbing against other ideas. Different belief systems, examined, show us the inconsistancies in our own actions or beliefs.
Victor Skaggs said This argument over Christmas mostly regards government, which is enjoined by the First Amendment NOT to express support for any particular religion over any other. That is why govt entities should not be saying "Merry Christmas" or displaying Nativity scenes, any more than they should be sanctioning (positively or negatively) Channukah, Eid, Holi or any other religious observation.
With the exception of a few die hards, this argument is about 2 generations past it's legitimacy date. There was a time when Christmas was openly celebrated by govts. in the US. That day is gone. The common culture has moved past it, due to successful lawsuits based on the recognition that public money or power should not favor one religion over another. We still have school winter break around Christmas. I think this is for practical reasons. Most people celebrate Christmas, many travel. I lived in an area where they closed the schools the first week of deer hunting season for the practical reason that so many students and teachers wouldn't be there.
The separation of church and state was originally to keep the State for either controlling the church, or to support one church over another (the State, after all is the one that controls the police and army). We've somehow twisted that to the idea that a church should not be able to express an opinion that may be termed political. This is historically inaccurate. Churches have always expressed opinions that conflicted with some peoples politics.
All that said, wishing someone a 'Merry Christmas', 'Happy Kwanza', etc. is similar to wishing them 'Good Day'. If someone doesn't want to have a good day, that's their own business, but I can still wish them 'Good Day'. Telling either the minority or the majority that they can't wish somebody a good day in their own way is just stupid and wrong.