People say the U.S. was founded as a Christian nation because the U.S. was founded as a Christian nation.
But that’s actually a complicated sentence that takes a book to fully unroll. And many books have tried to do so.
The shortest possible answer I can give is that many of the colonies were founded as religious havens, places where a sect could freely practice their religion - while treating all other religions badly. Ministers were both cultural and political leaders. They made the laws and the laws enshrined their religious practices, many of which last to this day.
After 150 years of this, the colonial laws got transformed into state laws, usually with few changes. It took some states decades to eliminate their state religions. Connecticut didn’t do so until 1818, Massachusetts until 1833, and New Hampshire until 1877. For much of this time non-Protestants were barred from holding office. Remember, the Bill of Rights was not seen as applying to state laws until the 14th Amendment.
What’s the other side to this? That horrible “D” word: Diversity. It was one thing to impose total control on a small, homogeneous colony, another on ever-growing cities with five-figure populations. And yet another level up to try to enmesh the often antithetical cultures of 13 disputatious colonies. Creating a country with one overriding set of federal laws meant that much of the individuality had to go. The differences between Quaker Pennsylvania and Catholic Maryland and Episcopalian Virginia were profound, and none of these wanted to be governed by the purported Christians in the others. The no establishment of religion clause added to the First Amendment applied to other states as much as it was a reference to the Church of England.
It wasn’t so much the religious beliefs of the Founders that set the tone for the country as their political pragmatism (and tacit hatred of others). None of them doubted that this was a Christian nation, if Catholics were counted as Christian, a matter that Protestants often disputed. They lived in a sea of Christians; only a handful, most of them in heathen New York, weren’t. But they didn’t want any particular sect to rule the others, a remarkably progressive stance for the day.
So. America was founded as a secular nation by a band of fractious Christians. That’s ironic. And it sure didn’t help non-Christians much for most of the country’s history. But the difference between “not much” and “not at all” turned out to be huge, much to the Christians’ chagrin, and it gets huger every year. Diversity, again, triumphs.