We never were.  Read the Constitution.

But Obama is ruffling some feathers on the right, because he actually said that at a press conference.

At a press conference in Turkey, President Obama casually rebuked the old chestnut that the United States is a Judeo-Christian nation.

"One of the great strengths of the United States," the President said, "is … we have a very large Christian population — we do not consider ourselves a Christian nation or a Jewish nation or a Muslim nation. We consider ourselves a nation of citizens who are bound by ideals and a set of values."

He's right, but it quickly led to a rightwing freakout.  John Hawkins of the Right Wing News:

Maybe it's understandable Obama feels that way since he spent twenty years' worth of Sundays at an anti-white, pseudo-Christian hate group instead of going to a real Christian church, but the majority of Americans, myself included, do consider this to be a Christian nation.

This country was founded by Christians seeking religious freedom and Christian principles shaped our founding documents and our culture.

This nation would not be a great nation without Christianity and it will not remain a great or moral country without the majority of its citizens remaining Christian.

That's not to say that there aren't great or moral people of other religions or even great or moral people who have no religion at all, but what's true of individuals, isn't true of nations. We've seen that played out in Western Europe, which is in a rapidly increasing state of decay and we're seeing it here in the U.S. as the number of Christians decreases.

The supposed "majority of Americans…. [who] consider this to be a Christian nation" notwithstanding, it is clearly not.

Of course, Obama isn't the first to acknowledge this truism. 

In 1796, and the Treaty of Tripoli was read into the Senate record by John Adams, and it was ratified without dissent.  The treaty read in part:

Article 11. As the Government of the United States of America is not, in any sense, founded on the Christian religion; as it has in itself no character of enmity against the laws, religion, or tranquillity, of Mussulmen; and, as the said States never entered into any war, or act of hostility against any Mahometan nation, it is declared by the parties, that no pretext arising from religious opinions, shall ever produce an interruption of the harmony existing between the two countries.

 Why did our founding fathers hate America? </snark>

I'll give Steve Benen the last word as a fitting response to Hawkins above:

We have a secular constitution that established a secular government. Our laws separate church from state. No religious tradition enjoys official sanction over any other. Of course we're not a Christian nation or a Jewish nation or a Muslim nation.

The usual argument is that most of the U.S. population is Christian. That's true, but irrelevant. Most of the U.S. population is white — does that make the United States a "white nation"? We also hear arguments that most of the Founding Fathers were Christians. That's also true, but also irrelevant. Most of the framers were also men — does that make our country a "man's nation"?