The Establishment Clause of the 1st Amendment makes this clear on the political side, but what about the people? Weren’t the people of the United States mostly, if not all, Christians?
Nathan Hatch in his work The Democratization of American Christianity highlights that the Christianization of America didn’t really occur until after the Revolution. We have the Second Great Awakening to thank for that.
Furthermore, while the United States was primarily Protestant for quite a while, no group really commanded a national hegemony. In other words, this Christianization was not a unified group of Christians; it was a plurality.
In the 19th century, the Methodists and Baptists commanded a majority of religionists, but they (especially the Baptists) did not have a centralized structure until a decade or two before the Civil War. In fact, according to scholars like Roger Finke and Rodney Stark, this very centralization and move away from massive evangelistic campaigns led to decline in the Methodist Church. They see this same phenomenon in churches that don’t evangelize, particularly liberal ones.
Hatch’s work also showed how the religious market in the States allowed by the Establishment Clause prevented one group from ever imposing its will on the country because there was too much competition. The freedom to exercise any religion allowed for explosive growth but not one established Church.
What would a Christian nation look like anyway? Is that dependent on sheer numbers or numbers of devoted followers? If the latter, how would you even quantify that?
On the sheer numbers side, church attendance has been steadily decreasing in nearly every church for around a decade. Denominations like the Assemblies of God have seen increased attendance, but this is primarily due to immigration.
What’s the point of saying the U.S. wasn’t founded as a Christian nation? I think it’s important to remember, because this phrase tends to be thrown around as a rhetorical device, particularly when a group sees its idea of Christianity being thwarted in the public arena. It can also be used to maintain the boundaries of a group that feels its ideals are in danger, not necessarily from outside forces.