The Supreme Court has ruled that this is a Christian Nation
There are many Christians who sincerely and even vociferously believe that America is a Christian Nation. One argument they offer on behalf of this is that the Supreme Court has official declared America to be a Christian Nation. Presumably if America is officially a Christian Nation, then the government would have the authority to privilege, promote, endorse, support, and encourage Christianity. All other religions, and secular atheism in particular, would be "second class" citizens.
This misunderstanding is based upon the Supreme Court's decision in Holy Trinity Church v. United States, issued in 1892 and written by Justice David Brewer:
These and many other matters which might be noticed, add a volume of unofficial declarations to the mass of organic utterances that this is a Christian nation.
The case itself involved a federal law which prohibited any company or group to prepay the transportation costs of a non-citizen coming to the United States to work for that company or organization, or indeed even encourage such people from coming here.
This was challenged by Holy Trinity Church, which had contracted with E. Walpole Warren, an Englishman, to come and be a rector for their congregation. In the decision, Brewer found that the legislation was overly broad because it applied to much more than it should have. He did not, however, base his decision on the idea that, legally and politically, the United State is a "Christian Nation."
Quite the contrary, because the things Brewer lists as indicating that this is a "Christian Nation" he specifically labels as "unofficial declarations." Brewer's point was merely that the people in this country are Christian — thus, it seemed unlikely that the legislators actually meant to prohibit churches from inviting famous and prominent religious leaders (even Jewish rabbis) from coming here and serving their congregations.
Perhaps realizing how his phrasing could create mischief and misinterpretation, Justice Brewer published a book in 1905 titled The United States: A Christian Nation. In it he wrote:
But in what sense can [the United States] be called a Christian nation? Not in the sense that Christianity is the established religion or the people are compelled in any manner to support it. On the contrary, the Constitution specifically provides that 'congress shall make no law respecting an establishment of religion or prohibiting the free exercise thereof.' Neither is it Christian in the sense that all its citizens are either in fact or in name Christians. On the contrary, all religions have free scope within its borders. Numbers of our people profess other religions, and many reject all. [...]
Nor is it Christian in the sense that a profession of Christianity is a condition of holding office or otherwise engaging in public service, or essential to recognition either politically or socially. In fact, the government as a legal organization is independent of all religions.
Justice Brewer's decision was not, therefore, any attempt to argue that the laws in the United States should enforce Christianity or reflect solely Christian concerns and beliefs. He was simply making an observation which is consistent with the fact that people in this country tend to be Christian.