Kevin Schultz a reader of this blog and a professor of Catholic Studies at the University of Illinois-Chicago, reflects on the fact that Americans have often understood themselves to be part of a Christian nation. He writes:
There is a long history to the idea that the United States is a Christian nation. The idea has come and gone with the prevailing politics.
The idea certainly didn’t first take hold during the revolutionary era. The first American patriots were not an especially unified bunch, and certainly not in the idea that the new nation should secede from Britain in order to create a Christian promised land. Indeed, there are 4,000 words in the Constitution and not one of them is god.
Instead, the idea that America is a Christian land was born in the early nineteenth century. Three ingredients helped create the idea.
First, there was a dramatic religious revival throughout the new nation, an event you’ll remember from high school as the Second Great Awakening. Interestingly, the Second Great Awakening occurred at least in part because the Founding Fathers who were evangelicals were denied from making the country a “Christian nation” in law. This denial opened the door for minority faiths, and the Methodists, Baptists, and variety of African American churches did just that, thus democratizing Christianity in the new country.
The second ingredient was the arrival of large numbers of Irish Catholics, those poor souls fleeing the Irish Potato Famine in the 1840s and 1850s. They were mostly unwelcome guests. Fearful of this impoverished bunch, many Protestants with longer standing American credentials declared that American democracy was only safe in the hands of a people with an unmitigated relationship with God. Catholics could not think on their own, and could not, therefore, be good Americans.
The third nineteenth century ingredient was stunning creation of America’s first truly national culture. Public schools, first developed in this era, began each day with Bible readings from the King James (Protestant) bible. Sunday laws were put into place. American literature and culture reflected Protestant values. America’s civil religion became increasingly Protestant.
And so the idea of a Christian nation was born. To be a true
American was to be a Christian and, more so, a Protestant.
For more on this entire idea, check out the first four chapters of my Was America Founded as a Christian Nation: A Historical Introduction.
RECOMMENDED READING
LONG FORM: Frederick Douglass and the Challenge of Seeing Clearly 
Is evangelical Christianity a religious movement, or is it something else? 
Anyone who wants to believe that Independence Day is a Christian holiday should read Frederick Douglass’s “What to a Slave is the Fourth of July?” David Barton speaks at First Baptist-Dallas. 
Bringing the Bible to the Jim Crow South
Whenever an author dates the Founding from 1787 and its “godless Constitution,” he's already stacked the deck.
America was already America by 1787. The Constitution is a political document after the fact, and as Dr. John Fea continually points out, religion was left to the states, many or most of which were already quite “Christian.”