"I keep hearing old people speak as if America is a Christian nation. Weren't we founded by deists, who deliberately kept God out of the constitution and created the "wall of separation" between church and state?"
A flippant answer might be to tally up the faith of US citizens, see that the majority are Christian, and grant "Christian nation" status on that basis. To the extent that the US explicitly does not have an established (state sponsored) church, your objection to the idea of "Christian nation" is correct. But the so called "Separation of Church and State" is a concept that has lately been woefully distorted. I rather doubt that anyone you've encountered harbors a wish that everyone could once again be forced to attend or tithe to the Congregational Church, or the Church of England (which is mainly what the "founding fathers" sought to avoid). What most people probably really mean when they invoke the concept of "a Christian nation" is this: They remember something different from today's rigid secularism. A state support for a Christian understanding of morality and for at least a vaguely defined sovereign Creator was a common experience of most Americans until the second half of the 20th century. Protestant Christianity was the zeitgeist, even in the government.
In my short lifetime I have witnessed a rewriting of the American story nearly as dramatic as anything seen in the history of nations. I recall something interesting from history (my memory being aided by a bit of fresh research, of course). An old photograph shows the Soviet dictator Joseph Stalin and a protégé, Nicolai Yezhov. Yezhov was the chief perpetrator of the notorious purges that occurred under Stalin, until 1939, when he fell out of favor. Then the horrors that he unleashed upon hundreds of thousands of others finally was visited upon himself; he was arrested, tried in a "show trial," and shot. Later versions of the photograph are altered so that Yezhov is no longer there. (Take a look for yourself here)
[Public domain photo; A relevant Wikipedia article can be found here )]
God is like a man expunged from an altered photo. Take the public school arena. My father grew up in an urban public school where not only was the "Pledge of Allegiance" recited daily, but so was the "The Lord's Prayer." When I graduated high school, the practice of an invocation (prayer) at the commencement had only just been banned. Look back to an earlier era of American public schools and you will find deeply religious texts, such as "The New England Primer".
Most of the "founding fathers" were Christian, though many (most?) may have been influenced to varying degrees by deism. These deists were certainly not today's atheists or agnostics:
On the surface, most Founders appear to have been orthodox (or “right-believing”) Christians. Most were baptized, listed on church rolls, married to practicing Christians, and frequent or at least sporadic attenders of services of Christian worship. In public statements, most invoked divine assistance. --Encyclopedia BritannicaModern skeptics seem to embrace this idea of deism as though it were equivalent to today's agnosticism, but this is wishful thinking. Deists at least believed in a creator, who set the universe in motion like the workings of a great watch, then stepped away. Their God was less involved in human affairs than traditional Christians would avow, but God was not expelled from the public square.
Consider the words of George Washington, whose "farewell address" is still read in the Senate:
Of all the dispositions and habits which lead to political prosperity, religion and morality are indispensable supports. In vain would that man claim the tribute of patriotism who should labor to subvert these great pillars of human happiness, these firmest props of the duties of man and citizens. The mere politician, equally with the pious man, ought to respect and to cherish them. A volume could not trace all their connexions with private and public felicity. Let it simply be asked, Where is the security for property, for reputation, for life, if the sense of religious obligation desert the oaths, which are the instruments of investigation in Courts of Justice?
Thomas Jefferson, who articulated the idea of "the wall of separation", was a deist, but nonetheless had no problem with the House of Representative chambers being used as a church on Sundays (See here for more information). His famous words of the Declaration of Independence invoke the idea of all men being created equal as a basis for action by the Continental Congress. An even earlier version states, "We hold these truths to be sacred & undeniable; that all men are created equal & independant, that from that equal creation they derive rights inherent & inalienable, among which are the preservation of life, & liberty, & the pursuit of happiness" (see here ).
The Constitution's precursor, the Articles of Confederation, mentioned the "Great Governor of the World." State constitutions uniformly mentioned God. "In God We Trust" was stamped on US currency since 1864 with few qualms expressed about this until the late 20th century. The US Supreme Court has retained the privilege of invoking God at the opening of every session.
This quoting of our forebears could go on and on. In fact here is a website that has listed many of them. An opponent of the Christian faith might reply, "so what? That was then and this is now." That is a valid argument for a different discussion, about what role should faith play in society here and now. What I am smacking down here and now is the preposterous claim of those who would say "that wasn't even there or then." The real story of America favors the Christian viewpoint. Of course, it has been said that history is written by the winners, and we seem to be on a losing trend right now, so who knows which picture will prevail. I, for one, will continue to stand up for truth as I see it.