It's not so much that the notion that Christianity and Christmas are being diminished in importance, more it's the method in which it is being accomplished that I am raising my concerns about.  We are definitely at a crossroads, religiously speaking, in this country.  I tried to do some research before answering and did, but everything is PDF, so I can only link and paraphrase.  I made some interesting discoveries.

There is no accurate way to view the religious makeup of the US from the beginning to the present, in the form of a graph, document or any kind of chart.  The US Census quit taking religious information over 50 years ago and the info obtained was not meaningful enough to make accurate conclusions.  The only thing that can be quantified with any kind of meaning is by church membership, that is those formally attached and attending a church.  There is no way to trend simple belief in this country until the last half of the 20th Century.

Church membership was very low in the country until about 1900 when it finally hit the 50% mark.  It seems to have peaked at the end of the 20th Century. Individuals declaring themselves "Christian" are presently about 75 to 80%.  That is an easily documentable number.  It is also a significant number as a constituency.  Christianity provides to some degree the moral compass of this constituency so therefore; social policies embraced by this constituency tend to be reflected in laws pursued by this "faction".  I'm not saying that it is right or wrong, just that it is.  It affects laws on social behaviour and is just as much a factor as say social behaviour laws promoted by environmentalists.  Some Christians are environmentalists and vice versa.  And sometimes there is an intersection of agendas and/or values.

There now seems to be a movement underway to make any values that are Christian to be dismissed as harmful to the good of the country and therefore less worthy of consideration as values put forth by environmentalists to keep with the comparison I am using.  Now we are at a point in this country where the values held (loosely, but legitimately) by 75% are considered to be invalid because they have a basis in religion even though they are consistent and congruent with the Constitution.  There is one national religious holiday in this country and it is Christmas.  It serves this faction of 75% of the population, yet it is a holiday shared by all as a national day off whether one believes or not.  The holiday is not officially constructed for only those who believe or adhere to it get the day off.  Interestingly enough there is only one national holiday devoted to an individual person, MLK. The rest of the holidays are for events or a group of people, former Presidents and Veterans.

We have a situation were the values of 75% of the population are systematically being thrown under the bus by 25% of the population for a whole host of reasons.  (This is broad brushed, but is there another way to present this argument ?)   And many of this 25% minority sees the Constitution as written to be a major obstacle to dismantling the status quo those seek to unseat or undo.  Fortunately, the Constitution protects religions as well as restricts them.  The Constitution was designed to manage and prevent factions from gaining an undo advantage over other factions, as anyone who has read the Federalist Papers is aware of.  It was also designed to prevent the tyranny of the few as well.  By having Christmas declared a National Holiday, has that meant that Christianity has an undo advantage over all others ?  Or was it a rational decision made based upon the fact that since so many observed it, that it may as well be given to all to prevent the feeling or position of legal religious discrimination ?  A cross that stood in the desert in California for the better part of a century honoring our fallen veterans was ordered to be removed.  If there, why not everywhere else including national cemeteries as they represent the same meaning.  There are easily many more similar examples if one wants to spend the time finding them.  These acts to me are a result of religious intolerance of the minority.  

I agree that many who proclaim themselves to be Christians are causing problems by proselytizing and trying to enter extremist interpretations into law of the land, and what is going on is a backlash to that end. Newly organized factions are trying to assert rights for themselves as an entitled faction regardless of legitimacy or legal standing.  They are trying to create a legal standing and are opposed by others who presently have a legal standing.  We are evolving.  Women were not originally considered citizens, nor were they allowed to vote.  Their present standing took almost two hundred years to achieve.  The impatience of these new groups or factions is propelling the attacks of the long standing factions.  Legal standing of these factions may come to be but that does not guarantee public acceptance.  That comes with the passage of time and the establishment of a consensus of acceptance.  Those seeking the to end the Christian dominance of this country are taking on 75% of the population and seeking an immediate as opposed to a gradual end.

What those seeking to unseat Christianity may be unaware of is that it is estimated that the majority ethnic group in this country will be Hispanic or Latino by 2050 and they are almost all Christian and mostly Catholic and much more strongly Christian than the shrinking white majority.  There could be an unanticipated backlash to the actions of today down the road.  Be careful what you wish for, because today's reality will be totally different in 40 years.

I may not have addressed all of your concerns, but this is how I am trying to explain my position that this is undeniably a Christian nation and as it looks come 2050 will still be, unless something very radical happens.

Here are the links to the 2 PDFs I mentioned in the beginning.  Of particular interest to me was the US Census bureau forms using the term "Christian" name and using the distinction of types of slave ownership including "corporate".

The other was for establishing religious trends.

We have come a long way from the beginning, it didn't happen overnight.  The pursuit of happiness was originally proffered as the pursuit of property.  I have learned in my short life that change that happens over a long time is lasting, while change that happens "overnight" or change for the sake of change is just as short lived and causes more problems than it solves.

We hold these truths to be self-evident,
That all men are created equal,
That they are endowed by their Creator
with certain unalienable Rights,
That among these are Life, Liberty and the pursuit of Happiness.-
That to secure these rights, Governments are instituted among Men,
Deriving their just powers from the consent of the governed.

In 1772, four years before the Declaration was signed, Samuel Adams wrote a short piece entitled "Rights of the Colonists as Men".  His words included the following:

Among the natural rights of the Colonists are these:
First, a right to life; Secondly, to liberty; Thirdly, to property;
together with the right to support and defend them
in the best manner they can.  These are evident branches of,
rather than deductions from, the duty of self-preservation,
commonly called the first law of nature.  All men have a right
to remain in a state of nature as long as they please;
and in case of intolerable oppression, civil or religious,
to leave the society they belong to, and enter into another.
When men enter into society, it is by voluntary consent....
Every natural right not expressly given up, or, from the
nature of a social compact, necessarily ceded, remains.
All positive and civil laws should conform, as far as possible,
to the law of natural reason and equity.  As neither reason requires
nor religion permits the contrary, every man living in or out of
a state of civil society has a right peaceably and quietly
to worship God according to the dictates of his conscience.

In case this excerpt is not sufficiently explicit concerning the origin of the rights so mentioned, further words from this same piece by Samuel Adams will make the point more clearly:

Just and true liberty, equal and impartial liberty,
in matters spiritual and temporal, is a thing that all men
are clearly entitled to by the eternal and immutable
laws of God and nature, as well as by the law of nations
and all well-grounded municipal laws,
which must have their foundation in the former...
The natural liberty of man is to be free from any
superior power on earth, and not to be under the will
or legislative authority of man,
but only to have the law of nature for his rule.^<1><2>

Peace,