I hear from the two older generations that America was a Christian nation back then. But the older I get, the less I believe this.
How was America a Christian nation back then? When did the change happen?
What's the evidence that it was not a Christian nation back then?
America has always been a predominantly/majority Christian nation, it still is.
But it's never been a Christian nation. The US Constitution forbids the establishment of religion; there is no state-sponsored religion or church. Early American legal documents expressly reject the idea that the US was founded as, or is, a Christian nation.
Because it has always been a majority Christian nation--most people here identify themselves as Christian--virtually all public servants and elected officials have been Christian, or at least identified themselves as such. Many of the early American intelligentsia were Deists, most famously Thomas Jefferson who subscribed to a Deist view of God and held to the belief that Jesus was a great moral philosopher (but rejected the Virgin Birth, miracles, Divinity, atoning death on the cross, resurrection, and all the other major doctrines and beliefs of the Christian religion). Some, like Thomas Paine, were quite antagonistic toward religion broadly, and Christianity in particular. But the common thread is that virtually nobody wanted a state-religion, and instead wanted to keep the US a secular state that secured freedom of religion to everyone without any external pressures by the state in dictating what people could or couldn't believe.
It is that principle of religious freedom that made the US such an attractive place for so many different people in other parts of the world who often struggled with practicing their religion in their own country. Such as many of the early Lutherans who came to the United States after the Prussian Union, a forced state union of the Lutheran and Reformed churches in Prussia to create a singular state-sponsored "Protestant" church. Lutherans, not wanting to lose their confessional distinctiveness, left to start new lives in other parts of the world, including the US and Canada. It's also why many Baha'is came to the US, such as my old neighbors, they had to leave their native Iran because the Iranian government is incredibly intolerant of anyone who isn't Shi'a Muslim (even other Muslims are discriminated against and persecuted there), Baha'is in particular are especially persecuted there.
It is that secular freedom to practice one's religion with liberty and freedom of conscience that brought immigrants from around the world, whether German Lutherans over a hundred years ago, or my Baha'i neighbors who sought refuge here just a few decades ago.
Now, as far as America and Christian morality/ethics is concerned. America has never been morally Christian--from its founding America had institutionalized slavery, and racism was baked into the Constitution until amended following the American Civil War. The murder and slaughter of the indigenous peoples is hardly in keeping with the biblical principles of justice and love. So from 1776 until 2023, there was never a time when America was morally Christian either. Systemically speaking, America has been--like all other nations and kingdoms of the world--quite un-Christian. After slavery there was Jim Crow, after Jim Crow there's still massive systemic racism and abuses of power baked into many aspects of the country that negatively impacts POC and other minorities and marginalized groups.
I can't think how, in any sense, America has ever been a Christian nation except in the sense that a majority of the population identifies itself as Christian. Putting "In God We Trust" on our money doesn't make the US Christian anymore than me in high school writing in my journal how hot I thought Jennifer Love Hewitt was made her my wife.
-CryptoLutheran